My system has always been able to run ~20 chunk render distance at a smooth and consistent 60+ fps, but yesterday all of a sudden I get huge lag spikes, slowdown, and FPS tanks if I have video settings higher than the minimum or a render distance above ~3-4, but the system goes nuts when I cross a chunk border. Some chunk crossings just reduce my fps for a second or two, but others freeze the game for a second. Tried fiddling with the RAM I give to the game, getting optifine, and with the video card drivers. None seem to work.
The spikes in the bottom left happen when moving across/loading chunks. Not even sure whats going on in the bottom right, as I couldn't seem to control it in any way.