What you describe is usually indicative of either a bad GPU (most likely) or a bad screen (less likely).
The only way to know for certain what the problem may be you should take it into your local Apple Store or an AASP and have them run diagnostics on the machine. If you still have AppleCare, any repair should be free. If you're no longer covered under AppleCare then you can at least get an estimate for the repair.
Just curious - which model MacBook Pro do you have (e.g. "15-inch Late 2011," or "8,2")?