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Originally Posted by Wizard
Air bubbles can only get from the headgasket. And other place where coolant would leak would be an external coolant leak. Not bubbles in the coolant.
Wiz
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This is not the only way air is introduced into the system. Granted he has a new waterpump on it, but if the seal goes out in the waterpump it will introduce all kinds of air into it with very minimal external leakage. The only leakage seen when the seal is bad (or the shaft of the pump is grooved) is after it's shutdown. And then it sometimes is only a tiny bit that doesn't make it all the way to the ground.
And it's not recommended to rev the engine with the cap off. The radiator cap is a two-way valve for that purpose. The system will push coolant past it at a set pressure. But when the thermostat opens, it draws coolant back into the system via the little poppet in the center. You can let it idle with the cap off, but use the techniques that Shane and those were talking about with the funnel to keep the level always above the engine so it is never starved for coolant.
I agree with Jet, eliminate the easy stuff first. Take out the thermostat and run it to see if it does the same thing. It could be that the thermostat sticks a little and then opens up, thus creating erratic readings. But it would probably be pushing the excess coolant out into the overflow. It could be just enough that it doesn't overflow out of the bottle before it gets sucked back in.