95E320 Engine Hesitation (M104 Eng)

My 1995 E320 with the M104 engine has an occasional hesitation problem upon acceleration.
The car has 139,000miles and runs VERY smoothly about 85% of the time.
Occasionally (about 15% of the time)upon a hard acceleration (with the throttle position beyond about half-way) the engine exhibits a stumble, and continues missing as RPM is increased....and 30 seconds later runs smooth as silk. If the throttle is opened quickly (for passing), I have noted a hesitation and "pop" (definitely an engine backfire, thru the intake). This backfire symptom indicates to me that the engine is temporarily way too lean!
I have checked the EGR system with a vacuum pump and have cleaned the EGR valve....works Fine. I also performed the EGR line (to the intake) cleanout procedure with a speedometer cable, that Steve Brotherton recommended...[this appeared to work through and clean the soot in the line, however, this work had no effect on the engine performance. This car has never had the engine electrical harness replacement......I was wondering if there is some problem related to the throttle position sensor and the engine fuel management system....due to the engine harness problems??????....causing occasional lean conditions.
Your advice on this issue is welcome!
Thanks,
Steve Whiteside