another noob question

I got an indication to stop on the dash the coolant temp was maxed out.  when i pulled over the car seemed fine and the coolant was full.  I would drive another mile or two and then the guage would rise to max again.  Also the AC had cut out right before it happened the first time. I have heard water pumps in these cars aren't that great but would it maybe be a sensor?

My frinds does the same thing, and yes it she sending unit.