Vintage Cars: 1986 930 loping/rough idle, engine management system, dual plugs

Question

engine pictures My 1986 930 is heavily modified - the drive train was rebuilt before I owned
the vehicle and has 23000 miles on it. The engine has dual plugs and was
built by a fellow who races porsches.  The car used to idle smoothly but has
developed a loping idle, that sometimes smoothes out transiently, but
general oscillates above and below about 1K RPM.  One mechanic suggested
that it was the "motronic" digital engine management system but I do not
have experience with adjustments to this system.  I would appreciate any
guidance you may have to offer about how to sleuth this problem out.  
Thanks.  Eric

Answer
This question difficult question to answer with out looking at car because of the heavy modifications. For example is this twin plug engine with crank fire engine using 3.2 intake and a idle stabilization valve.  If it were stock the answer would be easier. You need to find out what you have before it can be fixed. If you are local stop by.