If your head gaskets were blown, it could send pressure into the radiator, the pressure would be too high in your radiator, and your radiator cap should bleed it out to the overflow bottle. This doesn't sound like that. You aren't loosing any coolant, and your oil and water look fine. No system will be 100% completely free of air bubbles, so I wouldn't be concerned by an occasional small bubble. When your RPMs are increased, the water pump speed increases as well. Not sure if you have the cap off at running temp or not, but if you don't, the coolant really doesn't have anywhere to go so there may be more turbulence. Keep an eye on things and you should be fine!