i wont suggest you to update your motherboard bios if u dont know how to do it, and before trying other stuffs.
if it worked before, then it should work again without changing your bios.
because u can get your mobo fried if u do it badly.
i also have iw3mp errors, mostly when i just join the server, like 30 seconds after.
i think its related to sound, yes or pb
not sure what OS u have but cod4 was released before win7 and if u have 64bit u'll get even more problems with old games.
and i think we wont get any fix because well IW left cod and wont release any new patch.
and we can wait untill chickens got teeth before pb recode his bs program
now for me what works is that i join the server, then wait a few seconds, choose team, wait a few seconds then choose weapon.
all with a pause, where i dont do anything, and let pb do its bs.
that way my game usually wont crash
good luck
and yea maybe try what vovlo said.