White and Green Blocks and Tons of Lag

When playing on any texture pack, all of the sudden, all the blocks either turn green or white and when you move, the blocks start flashing (and extreem lag). If you exit and then go back in, then its fine for a few seconds, then it does it again. I tried to hit f2 to take a screenshot, but then it says minecraft has run out of memmory. But, i allocated it tons of memmory!