Lag and FPS are two totally different things.
If you are dropping frames rapidly, that will result in the so called "pauses". Meaning, if your frames drop from 110, to 30, that big of a hop will "Stutter" so to speak. This usually happens in combat, or when moving when more things are rapidly being processed.
1 way to fix that is to look at the hardware you have. CPU, and GPU are the most important for games, so if either is in need of an upgrade, i would do so. ( throw out that toaster, and upgrade to a blender!)
If your hardware is not that bad, then look for updates, update your GPU, windows, whatever, update anything that needs updating, make sure you have no background programs slowing your computers processing power, run virus scans to make sure there isn't anything malicious causing it etc etc. Common PC go to's.
Another way to fix that would be to enable V-Sync, this will make sure your FPS is only the refresh rate of your monitor, and make sure it doesn't go above that to eliminate stuttering and screen tearing. However, this will not work very good at all if your FPS is still dropping rapidly further than that.
Can you give me some more info? What does your FPS drop to when it happens? If it's dropping from 110 to like 60 or something, V-Sync will fix the stutter, but if it is dropping from 110 to something like 20, even V-Sync limiting your frames to let's say 60, a 60 to 20 drop will still stutter and slow down.
It could also even be a heating issue, or a bottlenecking issue. If your computer is running hot, it's gonna have problems. If your GPU/CPU are super unbalanced, like having a $500 GPU and a $50 CPU, you will get micro stutters if not second long freezes. Or if they are not compatible with the software you have it will cause problems. ( you can't combine a Ferrari engine with a $1000 Cavalier engine, it just doesn't work)
More info and i can help further :D
