Have you googled this problem? it seems to be fairly common.
Here's a couple threads/posts that could be worth your time, there's lot of possible causes and solutions.
http://forums.gearboxsoftware.com/showthread.php?t=79255
http://forums.gearboxsoftware.com/showpost.php?p=1936873&postcount=4
I have also read multiple times that surround sound system works bad with borderlands, setting it to stereo fixes problems, in case you have surround.