I agree. You're crossover cap is where you should be checking the fluid level. If you take it off, after the car is completely cooled down, and it is not full there, then there is air in the system. Add coolant there until it starts to add to the overflow tank (which you said is already at full level). Idle the car up to temp, which means the thermostat has opened. Once up to temp, give both upper and lower radiator hoses a good shake to help purge air out the crossover tube. If the lower hose is hot, then the thermostat is open. Do not remove the overflow tank cap. Don't aggressively squeeze the upper hose. That just gives the illusion that you are purging air. All you end up doing is squirting coolant out the crossover tube and onto your serpentine belt, which is not good for the belt. You might even draw air into the system when you release the squeeze.
Now let it fully cool again, which will take several hours. Check the level at the crossover tube again. Sometimes this takes a few times before you find it full. It took me a couple of days, and now my gauge is very steady.
