Plutonium

it only happens to me on some maps and seems to be only for zombies.
it happens after about 20 - 30 mins of being in game and its driving me nuts losing so much progress.
once it alt tabs itself it wont reopen it just alt tabs again if you try to tab back.
it seems there are a few people with this issue however I cant find any existing fixes, trying windowed just freezes the game around the same amount of time, and playing on lowest graphics settings doesnt change anything.