Every so often minecraft goes from 30 or 40 FPS to about 1 FPS. I call this a "lag fit" These "lag fits" happen every few seconds and last about 5 minutes. I originally thought that it was that my FPS was too high that my computer couldn't handle it, but lowering my max FPS seems to make it worse. It happens a lot more in the prereleases. If this bug is not fixed, my computer will probably explode. It would probably explode anyway though, it's really old.