I have a 1989 300SE and my problem is that there is a throttle response flat spot when the air temp is normal, 50 - 75 degrees. When it's colder out, I have good throttle response.
I did a search using these parms:
Search: Key Word(s): x11 ; Posts Made By: stevebfl ; Forum: Tech Help and child forums
or:
http://www.peachparts.com/shopforum/search.php?searchid=63771
which gives some valuable info.
Since a 'tune up' was done, I'd look for loose connectors or vacuum lines. This FI system is really simple, it's a CIS system with a computer wrapped around it. Fuel pressure is modulated by the EHA, electro hydraulic adjustment valve with the O2 sensor providing feedback. The EHA valve is at the base of the fuel distributor with a connector attached, or maybe not in your case. The computer is inside the car behind the passenger kick panel not between the two firewalls in the middle. That's the ABS computer.