well, per Diesel911's suggestion, I removed the "far right" wires (purple and white) from the terminal block, and tested each for current with the key off.
i DO get current on the white wire.
i DO NOT get current on the purple wire.

I guess this obviates the glow plug test mentioned in the last post...if I'm not getting power on the purple wire with the ignition off (and yet my starter is still turning), it's not the ignition switch's fault. right?

So, figuring it must be a bad solenoid, I reached down to explore that tight cavity so I could find a good spot to tap on it with a wrench. I pushed the solenoid wires around quite a bit, and then reconnected the battery without doing any 'tapping'. Suddenly everything suddenly seems fine! Starts/stops as it should.

Hard for me to imagine it's the wires, though...they have rubber insulation and a woven white insulation, and I can't get the starter to come on now by any amount of 'jiggling.' Also, how would bad wires explain the occassional "click / no start" condition?

Should I assume bad solenoid anyway, and chalk my recent luck up to 'luck'?

__________________
-lee
'83 300CDT (286k)

former proud owner of:
'85 mercedes euro 300TD
'80 mercedes 300TD
'77 mercedes 280e
'80 mercedes euro 250
'82 mercedes euro 250