Strange artifacts on my screen

Dear Hardware Gurus,

From time to Time, I expreience strange artifacts on my screen.
By artifact I mean some pixels which are ramdomly andrapidly changing color (mostly blueish). I don't know how to precisely describe the phenomenon, but I'd say scintillating ???

Most of the time, it appears in flash animations, but I've seen them in games or just on the good old windows wallpaper.

On my previous system (but with th same screen) I had experienced this problem too (and it was worse), but unplugging and replugging the DVI cable solved the problem once and for all. Now I have a new home-made system (the first that I did with my own little hands), and the problem is reappearing.

I have a Sapphire Radeon 5870 and a Samsung Syncmaster 226BW and a DVI cable to link them.

My system is not overclocked in any way.

What do you think is the source of this problem ? Should I buy a new digital cable ? Is my new Radeon already crashed ?

Thanks for your help.