Lag spikes occur, largely at random

Lag spikes occur, largely at random, I run at 800 FPS in this game, but occasionally I get massive lag spikes, so bad that the F3 screen cannot keep up and only registers a frame rate drop to ~400 where actually it drops, with no warning to ~1-2 FPS. I don't know for sure, but I've noticed it usually happens when a block's hitbox comes into view.

Duplication:

*play for some time in a snapshot