Before discussing the loaded misfire, I should place a warning to my previous post.
Be sure that when cracking lines loose, to catch the fuel lost with rags and don't let it accumulate. I also get chills up my back thinking of the people who will run right out trying to identify a ignition misfire with gas accumulating on the top of the motor. Be sure you have no spark arcs happening any where near. BE CAREFULL!
As to the loaded misfire. This unfortunately streches the tooling of many shops and most individuals. The technique becomes replacement diagnostics.
I have two tools, no one in my town has, to solve that problem on this car. the first is a programmable load chassis dyno. I can watch loaded misfires while sticking my head right in there.
The other tool and specifically the tool necessary to accurately test the current hypothesis: needs a fuel distributor. That tool is a differential flow meter. With this tool hooked to the injectors like a "milking machine" the fuel pump is hotwired and the airflow plate is held with a variable fixture to any depression including that which would happen at full throttle maximum speed. The flow rates of each cylinders injector/distributor path can accurately be measured and problems that will show up in performance are easily viewed. This tool is the factory tool made by Bosch and shown in early manuals discussing K-Jetronics. I know our local dealer doesn't have one. I'll wager very few do.
Without similar tooling its a guessing game, but I would start with injectors. I might also reverify ignition with a different tech. Ignition problems of misfire under load are real hard to see. In this case a portable secondary ignition scope is the answer. The best is the Vetronis MTS5100 (I know because I had the rest and just got the 5100). other good products are the Fluke 98 and the Interro PDA100 (got both of them). Find someone with the equipment and the experience!
Bosch Master, ASE Master, L1
33 years MB technician