As you have probably figured out, problems like this are hard to find and fix. I don't have a high confidence level in anything specific measure that I would recommend.

One thing to try would be to see if it would stumble while hooked up to an engine scope, a fuel pressure gauge, a vacuum gauge, a meter to check currrent to the EHA, and a meter to measure the duty cycle. But that would mean that a tech would have to literally watch all this stuff to see what was going on when it stumbles. You would need to find a shop with good test equipment, that knows the 560, and that relishes troubleshooting. You will also need deep pockets. And most likely, the car would not cooperate.

Probably the shop would get frustrated and recommend throwing a fuel distributor at it.

A do-it-yourself approach would involve self-diagnosis and judicious parts replacement.

So, first off, is there any pattern at all to the stumbling - time running, engine temp, ambient temp, after freeway driving, after idling at a stop light, after a hot start, AC on, yadda-yadda?

BTW, sounds like a great car, worth some time and effort to track this down.

__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe