From your brief description of the problem, its sounds almost certainly like a power suppply problem in the monitor. The easiest way to test it would be to grab a working monitor and hook it to your existing system, or take your monitor over to someone else's system and see if they are having the same problems.
LCD power supplies can be replaced (~$100, sometimes less) but from my recollection, MAG stuff is quite cheap and youre probably better off buying a new monitor.
But you can certainly stop swapping out video cards, as I really dont think thats whats causing your issue.
Hope that helps.
HooKs-
Have a MAG Innovision P900 LCD Monitor. Problem is the screen going black. Running XP Home on a computer (Wintergreen) bought from Tiger Direct last Christmas. Symptom is screen going black. Monitor power light remains green, moving mouse will not bring the display back, however if I cycle the monitor on and off display will come back for approx 2-3 seconds and then go black again. I have display and power settings set to Never for the monitor and hard drive as well as hibernate off. I did find the fan on the ATI Radeon 9500 video card had stopped running and very hard to turn, so instead of messing with ATI, I bought an Evga Nvidia based card, FX5500. Refresh is at 60 hz, resolution on desktop set to 1024 X 768. I know the MB is a cheapo and probably not the greatest with cooling, but that should not cause the display problems, so I am assuming the the LCD monitor is overheating. Any help would be appreciated. My next step is to swap it out with my used Gateway VX900 beast and see what happens, but would still like some input if possible. Thanks

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