TV tuner causing sudden shutdowns

I recently bought a video card and tv tuner for my 1 year old system. After installing both cards (video, then tuner), and enjoying the tuner for 2 hours or so, I realized an idle cpu load which caused me to investigate it's cause. I removed software, then the tuner card, then more software (ending up doing a system restore). I put the tuner card back in and found that this load problem was gone, but upon using the tuner card in Windows Media Center (thought I'd see how my 690G integrated would work with it), the whole system suddenly shutdown and required an off/on of the PSU to restart.

The next day I reinstalled the video card (HD 4670) and solved the strange cpu load issue by disabling ATI HDMI Audio. Hoping that fixed both problems, I tried the tuner again in WMC. It lasted longer (4-5 minutes), but shutdown suddenly once again.

Figuring it was a driver issue, I tried the driver version that worked the night before (the oldest of the three), AND tried the Catalyst Media Center this time, and had the same shutdown when scanning channels. There is also a noise that can be heard for a few seconds before the failure, like a fan or drive spinning.

If it hadn't failed with the video card out, I'd probably suspect the PSU the fault. However, here are my specs:

CPU AMD BE 2350
Mobo Gigabyte GA-MA69G-S3H
Memory OCZ Platinum XTC rev.2 2GB
PSU 500W Silverstone Element ST50EF-PLUS
2 HD, 1 DVD burner
Vista home premium 32bit
NEW Video card Gigabyte HD 4670 512MB
*the culprit!* Diamond TV Wonder 650 PCI-e

These components shouldn't be to much of a drain, with an energy efficient cpu, and the 4670 is pretty energy thrifty given it's power. And for it to happen with the video card out, I can't imagine the tuner would draw that much power in normal operation to shutdown the PSU. Any ideas as to the cause? Fried card? Could it not be seated properly (I've removed and reseated it and will test again tomorrow)? What about Catalyst Control Center, would having only the video driver be causing this?


Summary:

Worked for two hours: video card installed, CCC and CMC installed, oldest driver (came with 6.14.10.274), cpu load extremely high due to ATI HDMI Audio driver.
First failure: NO video card, all software removed, Vista recommended driver (6.14.10.283), no cpu load issue but ATI HDMI Audio driver enabled
Second failure: Video card installed, only drivers installed, including newer tuner driver (6.14.10.312) found on video card disk, no cpu load issue, disabled HDMI audio driver
Third failure: Video card installed, CMC installed, oldest driver (6.14.10.274), hdmi audio driver disabled, using CMC not WMC.