Problems with Dell Optiplex 990 upgraded to Windows 8

I have a 15 month old Dell Optiplex 990 (Core i7 2600 @ 3.4GHZ; 16GB DDR3 Crucial RAM (4 x 4GB); 1TB WDD HDD; 2 x Radeon 6450 Graphics dribing 2 x HP LP3065 displays at 2560 x 1600). Ran fine in Win 7 HP. Upgraded two weeks ago to Win 8 Pro and I get (a) video driver crash/restart after about 20 minutes then intermittently and (b) after about 30 minutes I loose connection to the Internet, even though the connection wizard indicates it's still up.

I have updated the AMD video drivers to the latest release (but not the newest beta) and reinstalled Catalyst control center; updated the System BIOS to A16; removed everything (HW & SW) that fails the compatibility test; swapped the memory around and tried it with only 8GB; run the diagnostics on the system, memory & HDD (no errors reported); Nothing has fixed the problem and I am about out of ideas. I am considering replacing the graphics cards with something from nVidia as well as installing new RAM.

Am I missing anything? Has anyone seen anything like this?

Might have to revert to Windows 7. At least I have a backup of the pre-installed configuration......