Problem solved! We cleaned the ground connection to the engine (it had a slight bit of corrosion), put on a new radiator cap (because the maintenance manual listed that as a potential problem), and replaced the sealing tape around the radiator that keeps the baffles in place and tight so that the incoming air goes through the radiator rather than going around it. It now works great. We climbed at maximum climb speed for 30 minutes at 90 degrees f and it stayed in the green. Yes, the old radiator cap was also 1.2 bar and did not leak or sizzle. Now we need to resolve oil temps that are too low because we have a big oil cooler.
