Hey Bill - that damn tractor is running right again--
It was indeed just a bad diode in the magneto harness. Actually very well documented in the service manual for these motors, so I can't imagine that the shop had actually spoken to BS about it. This diagnosis is on page 5 of the service manual, and when I asked about the diodes, the mechanic told me there were none on the engine.
So a pair of 90-cent diodes from Radio Shack later (actually 4 - I did the harness wrong the first time ) and the thing is back at 100%, ready to mow and blow. I faxed the shop the diagnosis page, as well as a detail of exactly what I would not be paying for.
As always--Thanks for the help and insight.
--Micah O'C
'17 M2 6MT, Mineral Grey
'04 330i ZHP
'88 M5 2791445
'92 M5T BL01001
formerly '90 535iM, '92 525iT