Well I had this problem on my ATi Radeon 9700 Pro card and I don't thing I need basic education in computers but anyway..
I did find 2 ways of solving this problem yourself (DIY). Its something that I've read at rage3d forums. I would ask that those that are having this problem with their FX5900 card try at least one of these ways...
1. Use DVI to VGA adaptor (some FX5900 card bundles will include this adaptor, like my MSI FX5900 did)
2. This is more dangerous one - use powerstrip & overclock your monitor past the default refresh rate at a given resolution. DO IT ONLY IF YOU KNOW HOW BUT ONLY AT YOUR OWN RISK!!!
Both of these ways solved my problem with Radeon 9700Pro and so they might work on FX5900...Please someone try it and report back, thanks