I have the same Radeon HD4890 and i have the same problems.
Roberto
Roberto, I heard about this back awhile ago when I was checking out the forums deciding which card to buy. Go to your video card manufactures website and not ATI's website and download a different driver package. I strongly suggest that you download the 8.60 package and unzip it to a directory on your system.
I read on some forums that this driver package solved a lot of problems and there's no need to go higher if you don't have to. You can always go up incrementally in releases if you want to later on.
Make sure you go to the control panel and uninstall all the ATI software. I don't think you need to remove the folders in explorer as long as you remove all ATI software. Then do a clean reboot and install the 8.6 package or whatever you decide to download. I would stay away from that ATI Smart Doctor crap. It screwed up my system and caused me to get a blue screen on boot up.