Vertex 4207 Transmit drops out when warm

[dmtemt]

Working on a 4207 UHF Mobile. First or second transmit are fine with a solid 40 watts on the meter... then it will drop to "0" watts and usually not come back on subsequent transmits... I can put a handheld next to the Mobile and hear a carrier given off on same freq but Bird meter registers nothing. I am suspecting finals are bad but when its working its solid as a rock... its all or nothing. Not sure if foldback circuit is flaky or what... its a pain to reach the finals and requires a hugh amount of disassembly and de-soldering... Any help or suggestions greatly appreciated... Dave