Finally looked at the o-ring area on the fuel heater and it's as dry as can be, with no hint of fuel leakage so I don't think that's the problem. The vacuum switch was also good because the stop lever remains in the up position when the engine stalls and the key is in the ON position.
I think it's time to start looking for a good MB tech in the local area. Not easy at the "end of the road" in Key West FL. Many thanks for your help and advice.

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