hiya m8 knightmare eh, sounds like there something running in the background in your system somewhere.
this causes the game to jutter, jump, stutter etc. also even if you did virus and spyware checks the spyware will not show up on your checks.
spyware hides in your pc and runs progs in the background which slow your pc up and cause connection probs as well, this will harm your pc more if its not top notch spec.
is your internet speed ok also m8 example when you click ukdirtwebsite up does it come up slower or just the same speed, check it with other sites u visit, if its really slow sounds like infected spyware, only way of curing m8 is to format your pc and start again.iv had to do it a couple of times,infact i may have to format myself after recieving that msn link last week which has invaded my pc with spyware.i knew there was something up when my car jumped frames and stutterd at arena essex in prac on friday.
try downloading ad-aware se pro nezza if your still having probs and see what comes up with that.it usually finds spyware stuff. that keeps your system nice and clean.
if it was working ok before m8 it defo sounds like something running as well in background.hope this helps m8