Well, it turned out to be my float heights. I adjusted for 23mm and it runs like a champ once again. Ran up to 90 and held it there for a while and no more stumble. It stuck me as strange. I rescued this bike and did a whole bunch of work on it before getting it on the road in 2010.
Question is: What would cause this problem to come on so suddenly?
Thanks for the thread, we're all in this together.
Off to the Sipapu, NM Beemer Rally ! (in the nick of time)