What causes ballpoint pens to write intermittently? After a while, a ball point pen doesn't write very well anymore.  It will write for a little distance, then leave a gap, then maybe write in little streaks, then maybe write properly again.  It seems to be worse with older pens, but I have observed this with new pens right out of the box too.
Experiments I have done:
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*Take the cartridge out and look at the amount of ink.  There is still plenty.

*Inspect the ball with a jewler's loupe, no obvious damage, everything looks smooth and clean.

*Stored new pens unused tip-down to eliminate gravity slowly pulling the ink from the ball and leaving a air pocket.  Some of the pens exhibit the symptom even when used the first time with the cap never removed before and stored this way for a year.

*Stuck a wire in the open end of the ink reservoir to see if maybe the end dried to a hard plug so that new ink couldn't move down as it was removed from the reservoir by writing.  I have never found anything hard, and observed the same symptoms even after "stirring" the top of the reservoir with a wire a little.

*When a pen stops writing, shake it hard, like resetting a fever thermometer.  That seems to help for a brief while, but so does just waiting a few seconds, so I'm not sure the shaking is relevant.

*Stored a pen ball-down in a glass of water overnight.  The thought was if the ink just above the ball had dried, maybe this would re-constitute it.  Some ink clearly dissolved in the water since it was colored, but once the pen was started up again there was no apparent change to the symptoms.

*Cold seems to exaggerate the symptoms, but warming to body temperature doesn't fix them.

This is not just a single pen or a single model.  I have bunch of different pens of different models that do this.  I'm curious, what causes this?
Added:
I have done some more experimenting, and it seems Emile Jetzer was right.  The cause seems to be that the ink is so viscous that new ink doesn't flow down to replace what is removed via the ball fast enough.  Two experiments support this hypothesis:
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*A pen will write again after a while by just letting it sit ball-down, but the time is significantly decreased when you shake it, like you would resetting a fever thermometer.

*Some stick pens are sealed except for a small air hole at the top.  Putting lips around the top of the pen and applying pressure as if you were trying to blow into it resets the writing action quickly.  Even better, I can write with such pens much longer than they would normally go by holding my mouth over the top and applying constant air pressure.

So, I think the mystery is solved.  Probably ink in the reservoir dries out slowly over time by losing water vapor from the top.  That makes the entire ink more visous, which explains why old but unused pens also exhibit this symptom.
The next experiment is to take such a pen and add a little water at the top of the ink reservoir, then let it stand for a week and see what difference that makes.
Added 2:
I added a little bit of water at the end of the ink reservoir in one of the problem pens.  I did this by using a small flexible tube (plastic insulation stripped from #22 wire) to put some water right at the end of the ink without a bubble between the ink and the water.
At first, there was no change to the symptom.  After about 2 weeks, the pen worked significantly better.  This pen had about 1 1/2 inches of ink in the reservoir, so it apparently took that long for the water to diffuse down to the ball end.
I think that this and the other tests conclusively prove that the problem is the ink drying out over time, which makes it more viscous, which prevents it from flowing down to the ball just by gravity as fast as the ball is capable of removing ink.
 A: Typical ballpoint pen ink uses benzyl alcohol BnOH as solvent, together with viscosity enhancers (such as fatty acids) which vary from one manufacturer to another. BTW, benzyl alcohol has anesthetic properties and will make your tongue numb if you get it in your mouth (yes, I was a kid once), but don't take this as a test advice.
Like any solvent, BnOH evaporates over time, leaving the ink more and more viscous until the symptoms you described appear. BnOH has moderate water solubility, that's why adding water helps the ink to regain some of viscosity. Another possible additive is mineral oil: it's also compatible with BnOH and has the advantage of staying there for much longer than water. Both water and oil (especially oil) will worsen the ink drying properties. Note that you'll want to keep your pen upright at all times if you add any solvent to it.
However, adding solvents will not keep your ballpoint pens working forever: given enough time various components of the ink will polymerize, making it a hard insoluble resin.
A: I would guess it's in part because of the viscosity of the ink. That would explain why the effect is more seen when it's colder. I don't know how doable it is, but you could try filling an ink cartridge with ink used for fountain pens, which is typically less viscous. You might get blotches of ink, but my guess is you won't get dry strokes. So maybe ink manufacturers used easy-flowing ink in ballpoints at first, but then saw it flowed too easily, and made more viscous inks. But this is speculative.
A: The air dries out some of the ink and seaps in through the cracks. Cold may exagerate the condition as it shrinks the ink and sucks the air in
so pens should be kept with tip in fluid I guess, so no air can come in beside the ball, not water cos it rots, not an evaporating fluid maybe, some oil perhaps, mineral cos bio goes rancid, 
A: Air behind the ball will do it. Get a good pen and write upsidedown and it will stop writing because the seal is not perfect. Check the ball and socket under a microscope and you may spot the wear on the older pens.
A: I think it's most likely that you are simply running out of ink and it's hard to tell in a typical pen cartridge. If that isn't it, it's a defect in the ball preventing it from rolling smoothly. 
You might not see the defect with a loupe. 
