The problem was that the previous owner had removed the distributer & slightly bent the metal windowed shroud. The shroud contacted the Hall sensor only enough to loosen the rivets. The contact was on the inside of the shroud, hard to see until you knock the pin out to disassemble the distributer. It spun nice & free. The sensor actually rocked when you pushed on it. I'm guessing that this heated up the Hall sensor causing it to stop working after 5 minutes. By the time I set up to measure the signal at the ignition module it had cooled enough & I had a signal.
I will sheepishly admit that halfway thru this odyssey I pulled the distributer & noticed the moving sensor. Thinking at the time that it was an odd way to fixture something (Doh).
I re-riveted it, put it back together & it now ran for 30 minutes before dying. The tach kind of still did a bit of a dance.
I put a new sensor in & magic no more problems. I will now clean the IAC, throttle body & check out other stuff to try & smooth out the idle.

Thanks all for your input.

ibs740