Need Help with monitor issue.

About 23 months ago I bought a refurbished Dell U2410 to replace my old LaCie CRT. I profiled it on my Mac G5 with the Spyder3 and after taking the brightness level to 0 was pleased with the result. It matched my target print nicely. Then last week, all of a sudden it was too bright and all the details in the highlights were blown out to white. I tried to re-calibrate it but to no avail so I assumed, with my good luck, the thing had failed just after the 90 day warranty had expired . I ordered a NEC PA241 with the Spectraview II and the same issue presented itself. I read over "Why are my Prints too Dark" again and re-calibrated about 10 times trying different settings until in frustration I turned it off. Yesterday morning I turned on the computer and checked my test print again thinking I would give it another go and the calibration that I had made the previous day was spot on !!! I thought that maybe something needed resetting and that turning off the computer had done the trick. Last night I was working and then was distracted so the computer went into sleep mode and this AM on restart the display looked too bright. When I checked it against my test print I found once again the highlights were blown out. Shutting down and restarting the computer had no effect this time.

The rest of the story.... I have an older Mac G4 under the bench so I thought I would hook the Dell U2410 to it and see how it worked. I profiled it on that computer and it looks fine... nice highlight detail!! I am a little baffled by this all but it seems the problem may be in my G5 computer. I am not that technical savvy...so is there anything besides a video card that would cause this to happen.