Volvo Repair: Volvo 940 (1194) rough idle but steady RPM and timing, fuel pressure regulator, 1994 volvo 940

Question
My 1994 Volvo 940 w/114k miles suddenly started running extremely rough in idle, only when stopped with transmission in drive or reverse, and only slightly better in neutral/park. RPM in idle reads 750 in park, very steady, and drive slightly less but still steady.  But the engine shakes a lot, never stalls.  It runs great at any higher RPM (moving or stopped)with no indication of erratic misfire or MPG decrease (20 MPG), all the way up through highway speeds, and tranny shifts fine. I think it's something to do with idle speed, but this model has no mixture adjustment and I can't obtain timing spec (reads about 7-8° on mark with #1 plug) to see if it's on spec. One mechanic mentioned defective motor mount, but wouldn't this cause a bigger problem at high speeds?  Plugs (Denso), wires (high end), dist cap and rotor replaced at 96k, air filter just replaced (no effect) and CV breather cleaned.  What could this be?  Is there anything else I can check?  I'm ready to change plugs, wires, cap, rotor again, but I think it's something else.  Thanks for your time, Larry

Answer
well gypsy cap and rotors can cause rough running. only get the porcelin caps from volvo or bosch. possibly the fuel pressure regulator is leaking fuel into the vacuum hose causing a rich mixture. motor mount is a possibility. and timing on this car is not adjustable.