Well, I don't know if the problem is fixed yet or not, but I be darned if I didn't find another vacuum line on the engine today that was disconnected. That's two now. Both were outside the circumference of the filter housing, so there's no way I can see that removing the housing would contribute to their being loose. There's no telling how long they have been that way. Oh, well.
I also replaced the idle speed control unit with one I bought off of eBay. Supposedly came off another 500SL with only 56,000 miles on it.

Drove the car several miles this evening and it seemed to run 'easier', but the idle never surged on it. Don't know if I drove it far enough and long enough to trigger the problem, but certainly will tomorrow.