Random Video Noise on CRT

Have you moved the suspected "bad" CRT to a different copmputer to see if the problem follows the CRT?

Have you tried a different video card to see if that solves the problem? Your video card could just be going bad.

Since you tried a different CRT and the problem continues, the problem is either with the video card or is environmental in nature.

Are there any large or powerful electrical devices, or power conduits in proximity to the CRT? Electrical devices can create powerful magnetic fields that can cause temporary (and sometimes permanent) problems with CRTs. High voltage electrical lines can cause the same types of problems. You might try moving the entire computer to a different location and see if the problems continue.

Hope this helps.

Look around the area for sources of electrical noise.

Motors,

Transformers,

High current power lines.

It may be as simple as installing a line filter (not just a surge strip) for the PC.

While a 'stretch', it is possible that the power supply or cooling fan in the PC itself is starting to fail.

Chas

The most likely cause of this problem is a bad ground in the wall plug.
If the ground is good, and the case cover is properly on, so it is also grounded, it takes industrial strength EMF for the the electrical noise to show up on the CRT.
By the way there is probably nothing wrong with the PC, CRT, or Video Card.
Bad RAM on the Video Card shows up as, not updated pixels, not as noise.
RAMDAC problems (Video Card)are very rare, but bad grounds are common. (Sorry about the pun.)
Start by checking the GROUND. (Earth, if you are British.)
Regards, Chris