FWIW, I've a 9800P that exhibited some of the flickering/rolling lines problems mentioned by some.
I'm running a dual-head setup, with the display spanned across one HP 17" and one NEC 19" monitor... I had flickering probs when running both displays only.
After some experimentation, I found that setting the refresh rate at 85 hz for both monitors was the best solution. I still have a slow rolling line on my 19" monitor, however, it's barely perceptable (depends upon application, background color, etc) in fact I only see it at all when using my web browser.
If it really tweaks me, I could always either turn off the left display, or separate the monitors (a little more space between them also solves the issue) but quite honestly it is solved well enough for me as it is. I really
have to look hard to even see it at all.
Anyway, I know there are many variables that could cause the same symptoms, but in my situation the advice offered by many here certainly was accurate. Looking for sources of emi around the monitor itself (in my case, another monitor, might be different for others) might prove to be worthwhile if you're having probs. Thanks to all here who contributed, I'm grateful for the advice