For underneath accessibility, I'd use ramps. Much safer than a jack/jack stand combination. PB Blaster has helped me with a lot of situations like this..ie..old exh. nuts/bolts. Spray on, let it soak over night.

This is a shot is the dark, but regarding the occasional up-n-down of the idle.
My 103 motor has been doing this on and off. Lately it's taken to stalling once underway for 15-20 mins. and in hot weather only -> fuel pump - nearly 16 yrs. old. When they start to go, the pressure can vary and you can begin to get a little bit of idle variation.

Hope to swap out the old Peirburg pump this weekend.

Good luck with the 104.

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