There is no magic on holding a set idle speed. It's just a closed loop of slowly closing the idle motor if above 775 RPM and slowly opening the idle...
I think he swapped that injector with one that was not spaying and the problem was still on that same line. That would point to the fuel distributor...
I always though my engine had the same "very slight misfiring pops". But I put some software into my idle ECU to detect a misfire which checks every...