Which model MBP do you have (15" late 2011, for example).
There are a number of things you could try - restarting in Safe Mode (start up with teh Shift key held down), resetting the SMC, etc., but I don't really think that those 'quick fixes' are going to help.
If I were you, I would just take the machine into your local Apple Store and have them run diagnostics on the unit. If it's out of warranty, they'll give you an estimate for repair and will charge you nothing for the diagnosis and estimate.
Sounds as if you've a hardware issue... maybe try running the Apple Hardware Test and see if you've any errors?