Extreme Framerate Drop

Basically, whenever I start up any game of MineCraft, FPS starts at around 150, or whatever crazy high number it should be, then it does this perfect parabolic slope DOWN to around 13-22 FPS which is NOT normal for my setup.
I have tried tweaking every setting, change java versions, deleting and reinstalling, older versions etc.
Might be an OS related issue though since I did just install Windows 8 today, but I feel like if I can run Battlefield 3 normally on here, i should be able to run MineCraft just fine.
The first time I noticed this bug, I had been playing the game for about 15 minutes and the games framerate had degraded to something unplayable like 5-8FPS and when I checked task manager it was taking 1.5gb of RAM, so at first I suspected a memory leak, but it doesnt seem like I can replicate that RAM usage. (I havent been able to play longer than 5 minutes however with this terrible bug)