I too have an M103 motor and recently posted an issue here regarding a drop in idle when the A/C was turned on. Here's the thread:
91 300SEL - Idle drops when A/C is on
My problem is very sporadic. I pulled the MAS and used electrical parts cleaner on and around the pins. Things looked a bit dirty and among other things, there is A/C control in this unit. To date, I have not experienced a reoccurrence. By the way - your M103 does not have a MAS. This device replaced the fuel pump relay/klima and a few others on 90 & 91 models like mine.
In another post, M.B.DOC mentioned items that affect idle. They are:
Throttle valve switch, decel switch, hall effect speed sensor; air flow position sensor. Most of these are covered by Steve Brotherton in the thread I pointed too above.
A simple place to start might be the ISC valve(idle speed control) valve. joe p mentioned this in more than one thread. This device controls air feed when the throttle plate is closed. In another post at this site, he suggested a simple way to clean it.
The cold start valve was mentioned earlier in this thread. I notice Mike that you are in DEEP south Florida. Steve Brotherton mentioned in another post some time ago that in x number of years, he's seen but a few failures of this device. The relevance is he too is in Florida and from what I gathered , this device might benefit drivers in REALLY cold places, but not Florida.