I have an '87 with the Dansk 1-in-2-out and 93 octane SW chip (and Fabspeed euro-style premuffler). I have none of the problems your reported, and did nothing special when I installed these upgrades. I did, however, install a new O2 sensor (purchased at a local chain auto parts store).
I don't have much to add to the very good previous posts except to point to the DME troubleshooting flowchart in the "tech info center" of this web site.
The obvious stuff are things like vacuum leaks, bad O2 sensor, bad plugs and/or plug wires, etc., are of course the first things to check.
In addition there are some AFM check/calibration instructions at
Air Flow Meter (AFM) - from "The 944 Motronic DME" by FR Wilk Yes, this is on a 944, but the instructions still apply.
Searching this board finds a LOTS of info on Motronic troubleshooting.
Good luck!
-Dan
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'87 black/black sunroof coupe w. f/r spoilers, fabspeed premuffler, dansk 1-in-2-out, sw chip, euro ride height, bumperette removal, CRD-220, etc.