[Amps] Amp Supply LK500ZB - strange failure mode

[k0zx Rich]

Wonder if anyone has experienced this failure mode with this or a 
similar amp:

This amp is 80s vintage and since then has been a solid performer, 
grounded grid with two 3-500z. But it has a developed a problem where at 
power up (no rf drive, ant relay input not active for tx), the plate 
current rises to about 3x normal (about 300ma, 100 is normal), stays 
there for 5 to 10 seconds and then drops to zero.
If enabled to transmit with  antenna relay input on, plate idle current 
is normal, going from 0 to 100ma.
If apply rf in, it will output but at only about .6 or .7 its normal 
output.
After inspecting the schematic am not seeing an obvious source for the 
problem.

Any suggestions appreciated
Rich