940 Non-Turbo

my 940 still shakes in the morning with RPM going down to below 500 for a second or two before moving back to 900. This happens when I step on the brakes and shifting the gear to R. Beside this, at traffic stop engine may intermittently idle rough but car does not shake

I've posted this previously and checked for vacuum leak as suggested on hoses, manifold, exhaust, etc but could not find any leak. my mechanic told me that valves need to be skimmed but a fellow volvo owner suggested that it could be the FPR. I've no tools to check fuel pressure but there is no fuel leak by pulling the vacuum hose attached to the FPR.

I've also cleaned the IAC motor and running it with a 12V batteries. The motor turns on contact so I suppose this may not be the fault.

COuld it be the brake booster pump because i notice that by pressing the brakes alone, the engine will idle rougher than usual. BTW, I've also changed distributor, caps and rotors. Could it be the plug wires? What is the OHM range for the plug wires anyway?

Any inputs?

Thanks everyone.

Laozong