The 1986 and 1987 603 engines are somewhat cold blooded when the glow plug cycle actually stops. A rough initial cold idle is the common description of this.

I have never had one of these engines. If I wanted to prove the problem. Putting a heavy gauge feeder to the glow plugs and leaving them on for the first few minutes of operation would be the test. If the rough cold idle left then I might upgrade the system to an afterglow set up as the newer cars have.

If that test did nothing perhaps looking for air getting into the fuel system when the car sits around might be my next step.

The idle speed is adjustable with a control somewhere near the firewall I believe as well. You might be set up a little slow making it even a little worse than it normally is.

All owners of these mention what you claim as well I believe. Hard to say if yours is worse than normal. Basically engine starts clean and then the idle starts to roughen up.