Really weird hardware issue, sound skipping/crackling under load

This one's got me stumped. My cousin's computer (i7 920, gigabyte x58 mobo, HD4850, not sure on ram or anything else) has been having this weird issue for a week or two:

Computer runs pretty much fine. Whenever audio plays from any source (media player, website, video game), it plays relatively fine. The second anything else happens on the computer, the audio will skip/stutter/crackle. This was on the onboard Azalia sound card. In order to try and remedy it, my cousin bought a new sound card (just a cheap PCI asus xonar DG). With this card, the same issue happens, but the sound slows down as well as crackling and stuttering. I checked for software and nothing suspicious was open. Eventually he decided he wanted to just reformat (Windows install was getting kind of murky anyway) and we did so. Clean install of windows with all the basic drivers, and the same issue happens.

It started happening randomly, never happened before. I've got no idea what it could be. Perhaps motherboard issues?

Are you sure the speakers are good?

Other than that it does seem like a motherboard problem, maybe something wrong with the southbridge.

Download the DPC Latency Checker and let it run while he's doing normal desktop stuff (including playing audio), and see if it shows any yellow or red bars.

If it does, start disabling components one by one in device manager to see what's causing excessive system polling. Most-likely culprits are LAN devices, USB, or audio. When he installed the Xonar it's possible that the audio chipset was still enabled, and if it's the source of the polling then it would still affect the system if it wasn't disabled.

Also, make sure the motherboard chipset drivers (Intel INF, and Intel RST if he's using AHCI) are installed.

Bump the Vcore a notch or two.

So before I got to look at it again, he did the DPC latency checker and it does indeed show red bars. I disabled the chipset audio when I installed the card, that made no difference. All chipset and RAID drivers are installed, so software is definitely out of the mix.

Will try disabling devices.