Simon - Thanks for the reply. Yes, the blower motor is working. I did verify the magnetron thermal cut-off is working (actually cuts off at about 170F - 180F Vs the 145F per spec). I did not verify the cavity thermal cut-off.
What would cause the relay to cut out? Most relays will fail to pick or fail to release. In this case, it will pick for 3 sec and then release.
Also, it seems that after I have worked on the unit, when I first power it up, all is good. I have run the unit for 10 mins w/o any issues (I put a few cups of water water in the microwave and it heats up just fine) It is the next day that it starts acting up again.