Perhaps, but you should also be aware that there is always variability in the GI, even when things aren't animated. The lower your GI settings the higher the impact of this variability. Depending on settings, successive renders of the exact same settings can still have differences, usually minor enough not to be noticed, but in some cases not so minor unfortunately. Higher GI settings usually solve this, or GI caching where appropriate (yes, even useful for still-frame rendering).
Also, although you may be seeing some *correlation* between the issue and a reboot having an effect, it is unlikely that there is *causation*, this is probably just coincidence. I can't think of why a reboot would change this, there is always variability (randomness) in the GI calculation (as well as other calculations, e.g. AA/ray-traced sampling). In other words I don't think it's actually correlated to any resource issue, which should not affect rendering in this way. If you run out of resources, rendering either crashes or gets really slow, it doesn't slowly degrade, at least not through any mechanism I'm aware of. But maybe Matt has some more deep-knowledge thoughts about it.
- Oshyan