so , i've tested multiple games running the same engine and reported  these things.

when you put your maxfps in 85 ( default and same as mw2 and mw3 ) you will get random mouse accel during the match. if you put it in a good sweet spot value /76 /125 /250 /333 the problem will stop.

i've did the same thing in mw2 (since it have the same problem) so , i tweaked my alteriwnet settings, com_maxfps 125 , and the problem stopped.

get your conclusions