1990 would be OBD I so not much help with reader unless there are some history codes in there
I would check the MAF(mass air flow) sensor closely, clean it, but also check the wires.
It reads like computer doesn't have a stable reading on air flowing into the engine, this could be from MAF not reading air flow correctly or from an air leak, in the MAF tube to intake or a vacuum leak.
Unplug TPS(throttle position sensor) and check its connections for corrosion.
If you have an EGR valve, with engine at idle remove vacuum line from the EGR valve, it should have no vacuum on this line at idle, if there is vacuum then EGR control module is bad.
Put another vacuum line on the EGR valve and suck on it, engine should start to stumble, if not EGR valve needs to be replaced and is probable stuck open a bit.
After(or before) any of this, unhook the Negative battery cable for at least 5 minutes.
This will cause the computer to Reboot at next startup, this erases and bad info as far as fuel/air mix and learned sensor data, so computer will relearn any changes you have made.