I have been having a similar problem to you. I notice FPS drops after about half hour and my dirty bomb sound slowly starts to “tear”, followed by all sounds on pc i.e. teamspeak and anything else I open afterwards. This is fixed by exiting the game.
I have not personally looked at my temps or CPU usage as I am running a fairly new pc: 4790k, gtx 970, 16gb ram, with all drivers up to date. That being said, it appears to be a graphics/sound driver issue.
I have tried to verify the game files through steam, and every time so far it has been missing 2 files, and downloads 6.8mb, probably has something to do with it. After verifying again today, and then turning vsync on (idk why i didn’t do this earlier), my problem seems to have disappeared.
I’ve only just signed up to the forums, if the problem returns I will take note of CPU/GPU usage and temps and post my findings.
For now i’d suggest verify your game cache and turn on vsync if its not already?