I've been getting random lag spikes on MC for at least 8 months now. My specs:
I'll be playing with at least 60 FPS, when suddenly I may drop to below 1FPS, for no apparent reason. It sometimes gets so bad that I just have to close Minecraft. I open it back up in the same place, and it can be fine. But if I lose close the world and open it back up anywhere, it lags, as if some memory is still being wasted from when it started lagging.
I usually play with normal render distance, no smooth lighting, and fast graphics. I tried optifine(heard it lowers lag), and nothings changed.
Any known reason for this lag? I think I was fine when I had my Radeon 6850.
Also when I first got my pc(i5 3450, Radeon 6670) I think I recall ~300fps on the best settings. I have no idea what happened, I have a much better gpu now, and lower settings, and I'm often seeing fps in the 50s.