rough idle Volvo 945T

93 volvo turbo wagon idles roughly. Mechanic (volvos only, excellent reputation, says ignition is missing on scope, but said he could not figure out the problem. At high rpm problem is not noticeable. He states tried air mass meter;coil and coil wire has missing spark signal to power stage intermittent tried ign ecu and fuel ecu-still problem.

A few weeks prior to this problem I noticed that idle speed decreased and car stalled when battery cable resistance was high. Burned my hand on battery clamp. Brushed terminals and battery clamps and idle returned to normal.

A few weeks later I donated my 84 Volvo and I replaced the battery of the 93 because my old Volvo battery was new. I have never observed changes in idle speed due to battery resistance being too high. I thought the car should run fine without the battery once it has started. I am an electrical engineer by training, but not an expert in auto ignitions.

Can anyone provide any incite as to why the rough idle? Would appreciate any advice.

Thanks in advance