Does the 3 mm allen wrench adjustment adjust the throttle plate?
 

On my 88 300 SE I'm having an issue with cold start fast idle. Twice a week it starts cold and runs at 900 to 1000 RPM and is fine, lowering to 700 RPM within a block, but most times, it starts and dies 3 times then starts & recovers from 500 to 900 RPM and runs fine. Is the 3 mm allen wrench adjustment at the fuel distributor adjust the air flap or the mixture? If I hold the throttle just slightly before the cold start it seems better. I've cleaned the ICV, replaced the plugs and wires, Replaced the OVP relay with the 2 fuse red one and made sure the 2 hoses to the ICV are not split or loose.

The adjuster to which you refer is the idle mixture adjustment.
It is quite sensitive; 1/8th of a turn (45deg.) can have a marked effect.
CW=rich
CCW=lean