Fuel injectors

1988 300 ZX, technical issue:
The car as approximately 270,000 miles on it the car runs great and uses little oil between oil changes that are ever 5000 miles, I have used synthetic oil in it for the majority of the oil changes.  Recently the car has started to miss after driving approximately 15 miles.  The symptom has been identified as the fuel injectors going almost closed.  The computer and air flow sensor has been changed out and the same symptom appeared about the same time.  I and my mechanics are at a loss to identify the cause.  Anyone have any ideas? I don't know if these tests were done or not, it was difficult to get the car to show any signs until it had been driven for a while.  The issue with the injectors was detected by driving the car with an oscilloscope connected to engine, not sure how it was connected but the mechanic is very competent.