Misfire

My 1985 308GTSQV just failed smog. High HC levels. The car wasn't running great but I took a chance and failed. I'm in Calif and cars are tested on dynos at 15 and 25mph. During dynotest, car started to buckle and misfire, test ended and showed high HC.This is logical because misfire would send unburnt fuel to the exhaust and thus high HC readings. Took the car home and during my drive on the street and freeway seems to run fine. However on the freeway if I am cruising on 5th gear, at about 3500 rpm, the car starts to misfire. If I accelerate, it goes away. Car seems to run fine except in this range on the freeway and thus on the smog dyno. I've since changed plugs, wires, fuel injectors, O2 sensor, and the car runs much better but still with same problem. Also at idle, it seems to be going up and down from 1000 to 1200 rpm and won't hold steady like at 100 rpm. I had a previous Mondial and it purred like a cat at idle, very smooth. Things I haven't changed is the warm up regulator, throttle position switch, fuel filter, Bosch injection ECU, ignition ECUs