Very interesting. There must be some other factor that's different between home vs field, or close enough that the temperature difference amplifies an exisiting issue.

Have you tried it at home, but outside? (To isolate between temp and something at the field location)

Have you tried detaching servos, one by one, at the radio? (To check for a servo issue)

Have you tried changing Rx antenna location, 90 degrees to long servo extensions?

Does it matter how the Tx is positioned when the flutter occurs? ie, does pointing the antenna in a certain direction or moving to a different position change the flutter frequency or amplitude?

Have you twisted the servo extension wires? (I'll probably get grief over this, but I will never again use extensions without twisting them first - it will cause events similar to what you describe, it has happened to me)