Windows 7 - Acer Veriton 5900 Windows 7 Sleep Problem

Hi all,
I have recently updated my Media Centre and am now running it in an Acer Veriton 5900. I have loaded Windows 7 64bit on it as the OS.
I have the Gainward Geforce GT 520 1GB SilentFX PCI-E card in my system. I bought it because of the low noise factor with no fan. It works well but I am having a problem.
I can put the Media Center to sleep fine, but when I wake it up I get no display for 1-2 minutes and when I do the Media Center interface runs like a snail. If i get out of the Media Center software Windows runs a bit better but struggles when having to do anything Graphic related. A restart of the PC and all is well again.
I have tried on another Acer Veriton 5900 with Windows 7 Pro 32bit and an NVidia 8600GT and have the same problem. The BIOS on the mainboard has been updated to the latest version from Acer that was back in 2008.
In the BIOS the only state that is accessible is S3.

Suggestions welcome as I would really love to be able to put my Media Center to sleep again and wake it up like I used to.

Thanks,
Dwarfboysim