U3011 via DisplayPort "forgets" it is Hi-Res, Drops to 640*480

Hi

I'm running 2 * U3011 (1 via DVI, the other via Displayport) off a Sapphire HD7950.

Both run fine for a while @ 2,560 * 1,600. The O/S recognises them as U3011's. Eventually the Monitor connected via DP "forgets" what it is & the OS no longer recognises it as a PnP Device. It drops the resolution to 640*480 & there is nothing you can do to convince it otherwise.

Problem exists on Win 2008R2 & Win 2012.

Is this a known bug with the DELL implementation of Displayport? Is there a firmware fix?

Other background info.

Occasionally I also swap in a U2711 (displayport) or a Dell 24" (DVI). I've seen the same issue with the U2711 when connected by Displayport.

I did find a workaround:

1. Connect the monitor via DVI.
2. Turn it off its power at the wall. Wait & turn it on.
3. Then the OS will detect it as a U3011 again.
4. Turn everything off. Swap the monitor back to DisplayPort cable again.
5. The problem is then "Fixed" for a few hours to weeks. But eventually it happens again.
6. NB: For me this workaround is annoying. The 2nd DVI cable is not long enough, so I need to unplug everything & move the computer just to get it working.

This process is why I feel that I've isolated the issue to the DELL product & not the Video Card, Cable or O/S driver. 

Thanks in advance for any thoughts & assistance you can offer.