Lets review where I'm at--testing done with a digital VOM when required

--HV loads check out good (capacitor, diode, magetron, HV transformer)
--Fan runs at high speed during cook cycle
--Switches good (monitor, thermal cutout, interlock)
--Power fuse good
--Thermistor good
--Temperature probe socket good


Symtoms review:
--Microwave system cooks/heats for about 40 seconds then shutsdown HV
load circuitry only (fan, magnetron, timer coundown)--everything else
appears normal (lights on, control panel displays timer countdown
seconds remaining in cycle)
--Cook cycle can be restarted immediately by pushing start
button--system will cook for another 40-50 seconds and then shutdown
again
--Microwave was tested with dummy HV load (magnetron, cap, and diode
disconnected) with same results--ie system ran for 40 seconds then
shutdown HV load circuitry indicating no overheating problem due to
magnetron.

Possible cause: Conrol or relay board.

Proposed action: (1) Replace board(s)--problem-none available
(2) Repair current board(s)

Suggestions anyone?