Monitor Problem...I think I'v narrowed it down

[SoCalRugger]

Alright, I've been having problems with my 2nd monitor (21" CRT) which is hooked up to the DVI connection on my video card via a DVI-VGA converter. After a period of using the computer, the 2nd monitor would become all wavy and go crazy, to the point where I could not read text on the screen. It's not the monitor itself, because the monitor works fine if I switch the connections. It's not the video card connection, because if I hook up my old 17" LCD directly to the DVI input, everything is fine. So I guess it's either a faulty DVI to VGA converter, or it's a problem with the VGA cable. Any ideas what would cause this? Or should I go out and by a replacement for both? Thanks.