I'll fathom a guess...
ECU problem or injector wiring problem. The motronic batch fires the injectors three at a time. You should be able to match the batch fire sequence to the ECU sequence; if they match I'd suspect the above.
You may also want to check the sensor wire from the #1 plug. That's the trigger for sequencing the firing batches.
BTW, how do you know what cylinders are not firing?
Let us know...curious problem.
Bellevue WA
90 535iM - not much stock remains. 3.7 liters, ported head, cammed, 3.73 diffy, M5 brakes, MAFed, yadda yadda yadda
86 Porsche 951 - Track Toy