Help! Lag problem

I have a lagging problem where I have a series of minor freezes, I have no idea why so can anybody tell me how to reduce the lag or get rid of it?

Current mc version?
Mods installed?

what are your specs? and how much fps do you normally get?

Try installing optifine and answering tedrocker's questions.

Is it when you press s?

1.7.9
No mods

when I lag at that spot, around 3 fps but normally I get around 15-25

Are there a lot of mobs, chests, or signs near you when you lag?

Is it at your home res or some place you usually go? If there's a where we need to know. I'll go check it out. It may be someone's giant redstone contraption or an animal farm.

Follow up: I seem to be getting 30-40 fps in one certain spot on your res, near the chests. I usually can get 150 fps. That may cause the frame drop. Do you have any redstone/farms under the ground?

It's actually on a different server, but it has a lot of plugins and has a moderate amount of signs, so it's probably the immense amount of mobs or something