Problem with 7800GT

When playing games(Call of Duty2 or Quake4) with anti-aliasing on, I quickly get small red horizontal lines across the entire screen. Less than a minute after this starts, the computer seems to freeze. I can bring up the task manager though and kill the game process. Most of the lines dissappear, and after a while if I refresh my desktop, the lines are completely gone. Normal Windows operations such as web surfing and watching videos have no problems.

I have the newest BIOS, Graphics Card Drivers, and patches for the games.

My BIOS hardware monitor reports the power supply voltages all very close except the 12V rail which varies between 11.71V to 11.77V, which is within spec for the power supply.

Could this be a power problem, or is it my graphics card?

My Computers Specs:

Windows XP Home
eVGA 7800GT CO SE
Intel Pentium D 830
ASUS P5ND2-SLI Motherboard
Antec TruePower II 550W Power Supply
2 512MB sticks of Corsair DDR2 RAM
150 GB Western Digital Hard Drive
Floppy Drive
CDRW Drive
DVD-ROM Drive