If you hit f3 it will show the memory going up and down extremely fast and lowers fps. It does not matter if you have an infinite amount of ram allocated it still does it. This is not a problem in 1.7. This is really dropping fps and causing problems for me in the 1.8 snapshots.
Please force a crash by pressing F3 + C for 10 seconds while in-game and attach the crash report (
[minecraft/crash-reports/crash-<DATE>-client.txt|http://hopper.minecraft.net/help/finding-minecraft-data-folder]
) here.