As noted in a previous entry, I recently bought my first LCD monitor, a Philips 190B6CB 19". It is connected to my stoic old Gigabyte Radeon 9200 via a 2m DVI cable.
I found that I got lots of sparkles on the desktop, which worsened to huge flickering lines when I watched digital tv via my PCI tv card. A sparkle is a single pixel that flashes somewhat randomly between two colours. The sparkle is colour, rather than location, dependent. If it occurs within a picture on a web page, then scrolling down that page will cause the sparkle to move down too.
Sparkles are relatively common in home-theatre installations where the DVI cable lengths are extremely long. They indicate that the signal is not strong enough to propagate the length of the cable. This can be cured by installing an active DVI repeater. In my case, the 2m cable was far too short for this to be of relevance.
As I mentioned in the previous entry, after some investigation I worked out that I had to untick 'alternate DVI operational mode' in the Catalyst digital panel settings. However, I also found that fullscreen 3D programs only worked when this option was ticked. Otherwise, the screen would go blank after flickering wildly on startup.
My thoughts were that if this was caused by hardware, ie. cable, video card or monitor, then it wouldn't be fixable by a Catalyst driver setting. An answer to this would be found if I could determine what that driver setting actually does, but I was unable to discover this.
As a test, I replaced my 1.5 year old $100 Gigabyte Radeon 9200 with my brother's 1 year old $300 Asus Radeon 9600XT, and found that this completely eliminated the visual errors. My current theories are that the latter card provided a stronger signal, or that its signal was more compliant with standards.
Update (14 Oct 2005): Setting the display size closer to the LCD's native screen resolution also improved matters somewhat. It's hardly a great solution, but might be a useful temporary fix.
Update (18 Oct 2005): After having major flickering DVI display problems when I upgraded my Linux partition from Mandrake LE 2005 to Mandriva 2006 I found a plausible explanation from a disgruntled user. Related bug entries in the X.org database are 1129 and 1829.