My 4870 had this problem too, it has to do with the firmware/bios of the gfxcard. Cause I flashed/updated mine and it worked great after that.
I suppose it's the Sapphire ATI card?
There was another way to fix it. Create a profile and edit the xml file to your 3d gpu clock, always use the profile and it will never switch back to 2d mode. Less efficient but it does work
"C:\Users\Your Windows ID\AppData\Local\ATI\ACE\Your CCC Profile.XML"