Sequential injector failure on 1992 Nissan 300ZX

Had good running car except at idle when only five cylinders fire. Replaced injector wiring harness on each injector, no change, replaced two additional injectors, third injector now failing. Car has 82K miles and ran on all six prior to touching injector wiring. Three injectors later, problem injector appears to be reappearing after plenum replaced, Help! Why cannot this be diagnosed properly? Corrosion on injector tabs not cleaned by technician which is difficult but better alternative than replacement.

Asked by for the 1992 Nissan 300ZX
The wiring to each injector needs to be properly tested, and repairs made if necessary. Sometimes trying to "fix" a harness can make it worse, and the entire wiring harness may need to be replaced to resolve this.