I have had a problem with the 3.2 M104 engine in my G for a while now. It started out as low power below 3500 rpm with a decided jump in power right at 3500. Now the low end seems even weaker and I have a stumble just off idle. Similar to an old V8 with a bad accelerator pump in the carb. I have checked the flap valve in the intake, the vacuum line is connected and the door opens when vacuum is applied. I was also thinking the throttle position sensor. This may explain the hesitation but may not cover the low power because the rpms, not the throttle position, cause the power fluctuation. Pretty technical I know, just fishing for possibilities.

bosch copper electrode spark plugs gapped to .040 inch (1mm)

It sounds like ignition to me. I would carefully inspect the plug connections and coil leads. It might be helpful to look at these in the dark and spray some water on the ignition at the same time.
Good luck