Located thermistor--results follow:
- Thermistor resistance reading was 275K ohms
- Substituted variable resistor in circuit for thermistor and ran
system operational testing for multiple values from 16K-2.2Meg ohms

--Result: Microwave ran for approximately the same length of time
before shutdown (40-60 sec) except for extreme resistance tests ie
extreme highs and lows then shutdown was 10-38 seconds.

Conclusions: Suspect control board problem--haven't located one from
suppliers yet--have have only found a control board varistor
replacement part for this unit. Assume a varistor is a variable
resistor--part photo from supplier looks like a small capacitor.

Comments appreciated regarding possible function of varistor in
circuit--are they possibly used in timing control circuits?