It's possible there's nothing wrong here. 2 factors mentioned are 80º and "when idling". Airflow through the rad under these conditions is very low so I'd expect the gauge to show right near the top. It doesn't sound as if it's boiling over. Running the engine above about 1500rpm should gradually bring the temp down when parked up like you are and driving at anything over about 20mph should do the same. I'd wait until you're driving to see what it's like. You've checked out all the problem areas and found nothing. Make sure you have the proper thermostat with the plunger underneath. Rad cap good? Correct length?