You mention a recent alternator rebuild. I hope the rebuilder replaced the voltage regulator while this was going on. Easy enough to check.

Elevate front of car enough to allow you to slide in from the bottom. Alternator on passenger side. Remove 2 phillips screws from the back of the alternator and pull out the voltage regulator. It has 2 brushes on it. Brand new, they are 1/2 inch long.

I've been thru the same thing and in my case, the VR brushes were badly worn down.

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Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"