I've been able to make some further testing with 2 other PCs. One is a laptop and I connected it to the u2410 with a VGA cable. There was no problem at all. Then I tried with a desktop (windows 7) and I connected it to the u2410 with a DVI cable. Again there was no problem whatsoever. As soon as I plugged back the u2410 to the Mac Pro the issue started happening again! I'm therefore inclined to believe that the problem comes from the MAC. I don't beleive it's a graphic card hardware issue but rather related to the operating system OS 10.6.4. Recently there was a Graphic Update from Apple but it didn't fix anything. I'm a bit lost here!!