FPS Issue

May affect other versions, please do inform me if this is the case.

What happens:
My computer usually runs Minecraft at 300 fps. Considering my computer's specs, it's not really surprising. Sometimes, however, when I start up a game, it just caps itself at 30 fps, and no matter how much I mess with the settings, it just stays at 30fps (my max framerate is Unlimited). It doesn't occur that often any may have just been a one-off, but still quite concerning.

How to replicate: N/A, is random.