[Elecraft] K3. FD shutdown

[Nr4c]

My K3 (s/n 453x) has been used for FD and other outdoor events plus use at home (till I  rec'd one of the first K3Ss). On at least 2 events in past year it has exhibited little output power. Batteries were checked, cables checked and replaced, to no avail. Display did not show SWR when TUNE button pressed (only "--"). Another K3 was swapped in and everything was fine. 
I took radio home and it worked perfectly. 

This year at FD, it quit at around midnight. Even tried <12 W to verify LPA. Result little or almost no power. Checked LPA screws but they were tight.  Voltage on radio was just above 12v on TX. Took it home and it shows no sign of low power. It has worked well all week. 

Reloaded FW from Elecraft and reloaded last CONFIG file (May 2017 just before first event of year and did TX cal at this time). 

Now hooked to dummy load and running stress test with FD CW msg on repeat. 

Anyone have ideas?

Sent from my iPhone
...nr4c. bill