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.
1971 Unimog 416