Got into it this morning, and went ahead and replaced the spark plugs since I'm taking it apart anyway.
The old plugs looked pretty decent (clean, with a light brown color), so I'm sure that wasn't an issue.
And it started and ran fine while I jockeyed cars around before I dug into it, and started up fine after the new plugs.
I will say that on the connectors to the fuel injectors, the wiring harness did have deteriorating insulation. It's cracked and brittle. No wires touching or anything, but it's not getting any better. I want to sell the car, but this is going to be a problem at some point soon.
And this problem DOES seem to show-up most when it wet outside, so there is a clue too......
Anyone know how expensive the fuel injector wiring harness IS for a 1994 S500??
Thanks for your help Brian, it's always good to have a clue as to where to look first...... :-)