Monitor displays distorted picture, Dell Dimension 2400?

[Hank M]

Every few hours or so my computer will freeze and displays distorted image of what I was just looking at. This usually happens when I'm browsing the web on Firefox. I don't think this is an overheating problem because the CPU, GPU, Onboard Graphics etc. all seem to be at regular temperature when this occurs. I have to force shutdown the computer and reboot to use it again. As precaution I bought some new thermal paste for the CPU to replace the old dried up. I have purchased a new IDE video card as well.
Here is some links with more information:

Picture of Computer Screen: http://i1090.photobucket.com/albums/i370...0e88f4.jpg

Computer Specs (Dell Dimension 2400): http://support.dell.com/support/edocs/sy.../specs.htm

New Video Card: http://www.ebay.ca/itm/New-ATI-Radeon-75...2c62b225a2

What may be causing this issue? How do I prevent this from happening?

Thanks in Advance!