Loss of Video

I have a computer system that does the following:
I turn on my computer (cold system) and within about fifteen minutes the screen goes blank. I turn the computer off let it sit for a few minutes, turn it back on it’s fine for a about a half an hour, then same thing “blank screen.” I have changed video cards, disabled screen savers, power management, I haven’t a clue what to do. I’m running Windows 98, with a standard Trident 4MB Video Card, originally the problem started with a SIS 630 8MB Card