CA20E "Limp Mode"

[200sxowner]

I have a 1984 Nissan 200SX that recently developed a problem where it occasionally goes into "limp mode". To this point I have changed the plugs, wires, distributor cap, rotor, O2 sensor and even substituted the throttle position sensor and airflow sensor from an earlier model that had been running. Nothing seems to correct the problem and as I understand it there are only two more possibilities, and both are costly. The Ignition Control Module and the ECU. How can I determine which to spend the big bucks on to correct the problem, or is there another possibility. I am 120 miles from a Nissan service center and Checker says there is no way for them to hook-up their diagnostics in this early of a model year. I don't want to scrap it since I just put in a new rebuilt engine some 50K miles ago and it gets great gas mileage (when not in the limp mode).