What version of Realism are you using (and SPT Aki version) and is this a recently issue that has popped up with an update or change of game version or is this an ongoing issue that you are having?
Ultimately this is VERY unlikely to be directly related to Realism or any of the changes that it makes, it may just be that Realism is causing more of a performance impact on your system. Generally instability such that you are describing may be due to overclocking, is your ram, cpu or gpu overclocked? If I had to guess it would likely be related to ram.
He's running 3.5.5, but also had an issue with it on 3.5.3. I agree that it likely isn't the mod itself as he and i have the same modlist, however, we have been unable to troubleshoot it past it doesn't crash without realism, but does with it. I also thought it might be his ram, but in watching his task manager, the ram never caps out. I don't have answers on the overclocking, but i'm fairly certain that it isn't. I do agree that i think its either ram or a potential dual port collision on a cpu thread, even with task manager never showing it capping out.
Is there anything else you can think of that it might be though? I've had him try and look at most everything i can think of minus a windows reinstall as i know I've had a few issues with windows randomly deciding that it didnt like certain games and either crashing them or not allowing them to load at all.
Planning to look through the Bepindex logs tonight as Fontaine suggested. If we cant make sense of it, i recommended he post it to let yall look too.