Well, today I decided to actually use the supported refresh rates of my monitor. The only trouble is the DCC chip is a bit nackered on it (well it doesn't work) and it's a bit dodgy with ranges.
For example, after tweaking this 19" Pure Flat CRT baby, the max refresh at 1024x768 is 119hz (120 takes it out of horizontal scan rate range!)
Anyway, my secondary monitor, a measly old 15" was shimmering a bit once I'd upped the frequency on the other (both originally ran at 85hz.)
It's now running at 89hz so that it's sort of in synch with the other - no more flicker.
And after all that - I can barely tell the difference, lol.