Intermittent problem

I am having an intermittent problem, on an 1990 Chevy P/U Truck 350 two wheel drive. When I start it in the morning it idle up and down then smooth out. After it warm up it idle rough and just one injector is working. Change electronic module under the distributor, O2 sensor and checked pick up coil for proper resistance OK. What I did was unplug the ECM connector and plug them back it seem to work for a while, but problem come back unexpectedly. Any information would be greatly appreciated.

might wanna check your distributor shaft for play

The ECM controles the injectors. If 1 injector is not working its a bad injector or wiring to the injector or the ecm. Most times its bad ecm quad driver.

Since you said you unplugged the connector from ECM and then reconnected it as an attempt to "reset" itself which worked for a while, I'd agree w/ Fireplug. My initial thought was bad connection in the wiring if not the injector itself. But if you got it to work again and the ECM quad driver is most common problem according to Fireplug, that would be the bad connection