It seems to be caused by a memory leak, with CPU usage nearing 100% even at relatively quiet areas of the game. Multiplayer modes aren't free from these issues either.
Treyarch are looking into the issues, so we expect a patch to be issued shortly. You can watch a clip below to see an example of the lag and stuttering that has been causing frustration for many gamers.
PC Gamers suffer with Call of Duty: Black Ops
10 November 2010 | By Jamie Davey