After about playing for a day or two when my system manager memory usage hits about 600 mb for minecraft the game will freeze for a second about every 5 seconds. Exiting and going back in fixes it for a time but it always happens, and it takes an annoying amount of time to load back into minecraft each time.
Running buildcraft/Industrialcraft^2, only affects me in singleplayer, multiplayer works just fine (for buildcraft anyways), working on setting up a mp industrialcraft server for additional testing.