DRZ Cooling not so cool

I'm suddenly getting way too much heat off my DRZ 400 motor.  The engine oil looks clean, no anti-freeze in it.  The fan works. The radiator is full.  The overheat light isn't coming on so I must be getting some cooling. Anyone else encountered this problem?  I'm suspecting the thermostat or waterpump but I'm looking for other suggestions or troubleshooting ideas.  I would like to get it fixed right the first time so it's ready for the Heart of Idaho ride.

Gary