Problems doing video capture with SB81P and VIVO

I have an SB81P that I want to use as a PVR. As there's no room for a dedicated video capure card, I thought of using a video card with VIVO. I've tried two, but both are having problems. Both cards are Gigabyte. I've tried getting support from Gigabyte, but they say they think the problem is wiht the motherboard. Of course, they would say that, but I'm beginning to wonder if they might be right.

The first card is a GV-NX66T128VP NVIDIA 6600. It seemed to be good, but always falls over after about 20 minutes, bringing the PC down with it. (This card isn't really suitable for the PC anyway.)

The other is a GV-RX70P256V ATI X700. It doesn't fall over, but when actively capturing video the input is slightly, but noticeably, jerky, as if it's using too much CPU.

The PC has a Pentium 4 3.4 Ghz, 1GB of RAM. The cards are PCIe, of course.

The fact that the cards exhibit entirely different problems makes it hard to know if it's the motherboard that's the problem. Unfortunately, I don't know enough about the technology to know what the problem might be. But there's no point in returning the cards only to find that they all have problems.

Needless to way, I've updated the drivers, tried various graphics settings, etc.

So I'd appreciate any suggestions as to whether a limitation or setting related to the motherboard is the problem.