Update dec 2012:
This issue also exists in Black Ops 2 but it’s a bit more subtle/rare – the fix is the same though!
I used to have some problem
with the mouse sensitivity in MW3 skyrocketing and then being normal and then skyrocket again and so on.
I read on various forums that turning off FRAPS would help, only problem is I dont use FRAPS or have any software like that running. Also read that lowering the pulling rate on my gaming mouse would help – I did not care for this since its one of the reasons I bought it. However that did help around under 250 Hz. Also read tips about lowering process priority and so on, nothing really helped.
So what did help? Well I think one of the reasons this bug is so hard to pinpoint has to with the fact the game is running under a high priority and needs to be doing this to work well in mp.
The solution lies in killing all oher non essential processes. In my case Dropbox is usually the “culprit” (not really though, it’s MW3 that is shitty but you get my drift).
So if you are having this issue, try killing off non essential processes one by one w/ the task manager until game is normal. Re-installing Windows will do nothing. Trust me.