I had that problem with radeon x1500 and x1600 so I supose the solution will be the same, and you must be using de mesa driver (the open source). I don't know if it works with the propietary fglrx (didn't for me...)
Reboot, and in the grub screen where you select the OS, pres "e" over the first entry. You will acces to a manual editor for the boot command.
Go to the end of the line with the linux kernel parameters. At the end it must be something similar like "ro quiet splash" (depending on your boot settings). After the last word, and with a space between, tipe "pci=nomsi" (without the "") DON'T PRESS INTRO, because all the orders must be in the same line... if there's no space when you write the last part, it will appear down but it's considered all the same line.
Pres control+x and you'll bot with the new parameter. Try activating compiz, running some games in 3D etc... If it doesn't work don't worry, you didn't change anything and the next time you boot the parameters will be the originals.
The problem is everytime you reboot you have tu do that, so if it worked you can make it permanent installing Grub Customizer:
sudo aptitude install grub-customizer
Open it from apps, system utilities, go to preferences, general, and in the kernel parameter box put "pci=nomsi" (again, without the ""). Close, save the new parameters and reboot.
If there's any problem, just run the program again and delete the new parameter.
I wish it helps. Good luck!