92G20-acts like it's hitting on 2 or 3 cylinders only!

167k miles on my '92 G20. Meticulously serviced. Allowed brother in law to drive for 2 months. Came back complaining of "no power anymore." Sounds & feels like it's coming apart - motor mounts may break any moment from the vibration - 2 to 3 cylinders only. Hitting on all 4 cylinders. Plugs, wires, injectors checked. Possible ECU? Any help would be appreciated. I have gone thru the "self diagnostics" in the service manual.

Thank you,

Elerator