U2711, strange power saving and OSD freezing issue...

Hello! I recently bought a used Dell U2711 to include free with the following project I've been working on (I was going to send a Dell U2412M, but really wanted the recipients to have a higher res screen for working more easily with HD material):

www.sgidepot.co.uk/.../charitypc1.html

When the display is up, it all looks ok, but issues arise when I turn off or reboot the system. It fails to recognise a signal after any amount of time away from being fully on, ie. the power saving mode goes haywire. After shutting down the PC, instead of the power indicator lighting changing to flashing orange, it will either change to steady orange or steady blue and then in either case get stuck (ie. power and menu buttons have no effect). Likewise, with the system up and an active
signal, if I try switching the inputs via the OSD, the OSD itself can just freeze, ie. lock up. It's a pity because the picture itself is fine, but atm it can't even handle a reboot - screen stays dark, kybd
lights indicate normal booting, but nothing on the display; if I unplug the power cable from the monitor and plug it in 20 seconds later, then the screen will come up ok, ie. to the Windows desktop. Trying the connection on a different monitor (Dell U2713HM), it all works ok.

The PC is running Win7/Pro/64bit, GPU is a GTX 980. Note I've tried
both via DP and DVI, using new cables (DVI was dual-link), but no joy,
same thing happens.

Here's a picture of the screen while the screen is working ok:

www.sgidepot.co.uk/.../P1120738s.jpg

Anyone any idea what might be wrong? Perhaps faulty power circuitry?
I tried a factory reset, no luck.

Thanks!!

Ian.

PS. The Service Tag on the unit is <Service tag removed>