Suspect the bias diodes. It isn't uncommon to get a bad internal
connection in one. Replace them, even if you have to fabricate your
own from discrete parts, don't believe a meter.

To check the diodes, it helps to have some current flowing ~100 ma and
then apply heat and cool to them. The voltage change should be smooth
with temperature.


Ditto the diodes. Also, there has been a number of fake, counterfeit
transistors sold in recent years. If your MJ series did not come directly
from Motorola, Onsemi, or B&D Enterprises they could be fakes. You can
Google the topic of counterfeit transistors for an eye-opener. I've had
exactly this happen. New parts with NO other circuit problems, fail within
minutes when playing into a load.

Mark Z.