A member on the site once had his radiator caked in mud and that caused it to overheat quickly... Paul in Moab, Tone helped take the radiator out and clean it.. and the reinstall, fixed the problem

I've gone to the H quickly in 90 degree weather in the sun just sitting with the air on (went into the store with, left the dog in the A/c) it was close.. got out there just in time to drive and drop the temp... have to be moving for the car to cool itself.

Just some thoughts