05 rx-8 heating problems

I have a 05 rx-8 with 81k miles on it. I just bought it today. When driving today heat worked great. When driving a few minutes ago I drove it for about an hour and heater was on and heat was hot then cold then hot then cold. Car was not overheating the gauge stayed exactly where it needs to be in the middle. Car ran execellent and there is coolant in it. I'm starting to think there is an air bubble stuck in the heater core. Basically just wanted someone else's opinion and knowledge on the best way to get rid if the air bubble if it was to just drain the whole system and start fresh? It's a pressurized resivour and I didn't see any bleeder anywhere so it doesn't seem very easy to bleed.