Minecraft random performance?

Checked and there was no megathread to ask this question. I play Minecraft 1.6.4. So yeah, old version. I am able to get GREAT performance on this version, with no issues at any point, pinned at 60fps at any point. However, I can reboot the game and it will randomly be at 15fps. I have the game set to only take 3.5gs of RAM, it's set for 32 bit and is PLENTY for vanilla. However I will randomly reboot the game and it will be running like absolute shit. What do you guys think?