Can pair production and recombination make a gamma ray appear to travel slower than the speed of light?

I was reading about pair-instability supernovae, which is when gamma rays produced in the core of a large star become energetic enough to produce electron-positron pairs, which causes a loss of radiation pressure.

Am I correct in assuming that if a gamma ray of sufficient energy were traveling through free space it would oscillate between being a gamma ray and being a (perhaps virtual) electron-positron pair? Would the time it spends in the latter form cause it to take longer to get from point A to point B than expected from the speed of light?

According to this question Is pair production only with $\gamma$ photons? it requires another body nearby such as an atomic nucleus to exchange momentum with. OK - so suppose the gamma ray is traveling through space, but there are a few atoms per cubic meter of ionized gas. Would the effect occur then? Is the result different than the speed of light through the medium or the effect through scattering? (I.e. difference between actually bouncing off atoms vs. passing near atoms.)

• It is said that gamma's produced inside the sun's core take about 1 million years to emerge at the surface, but that's more of a figure of speech. It's not the same gamma that comes out, but thermal radiation that was always in very close equilibrium with the surrounding matter, and that's the key. Radiation, in equilibrium, doesn't "travel", at all, it's basically stuck stochastically in one place. Only if there is a thermal gradient do we get a heat flow. Jun 15, 2016 at 20:16
• A photon needs an interaction for pair production, otherwise conservation of energy and conservation of momentum cannot be both conserved. Therefore, a photon traveling alone cannot produce a particle pair. Jun 15, 2016 at 20:17
• In the context of QFT, you're asking about the one-loop mass renormalization of the photon. It can be shown that there is none, i.e. the gamma ray will travel at $c$ even with this taken into account. Jun 15, 2016 at 20:50