Now for the bad news.....
Last night, while doing nothing in particular, the screen went blank, and the green indicator light on the monitor turned back to orange, but it was obvious the computer was still running fine.
A reboot, and all seemed fine.
Tonight, I installed Rage3D-tweak 38c, so I could pump up the memory.
Bumped the core up to 285 and ran 3DMk02 once. Similar score to stock. Right-clicked the desktop to push things a bit further, and the screen froze like the initial problem, for a couple of seconds, then sure enough, dumped back to low res.
Reboot, and right-click the screen to reset to default speeds, and the monitor tabs all stay white and do not fill up, and the screen is locked. Dumped back again.
Into safe mode and a successful uninstall of Rage3D and that's where I'm at right now.
Any further thoughts??