1985 Nissan 300ZX cutting out

I have a 1985 Nissan 300ZX V6 3000 170000 miles. The car is cutting out, not like a miss, more like a rev limiter
at approximately 1800 to 2000 rpm, car will start jumping or hesitating. in park it will rev to approx 1800 rpm them
act like it trying to shut down, if hold throttle keeps doing same thing, if give more throttle wont pick up even if
go to floor still just comes and goes. Have had two different mechanics on it, have replaced distributor-complete,
mass air flow sensor, throttle posisition sensor, fuel pressure is 60 PSI, new plugs and wires, this thing driving me
nuts, can only go about 40 mph on interstate. thank you for any help or suggestions.Dave thanks for the comeback, but might have misled this problem, it is more like a serious surging, than a misfire, have checked all above plus the converters, dosent actualy miss or loose power like fuel, just surges real bad and wont throttle above 2000 rpm without acting like cutting key on and off. any help is appreciated.