A 1990 is the last year with an EGR- probably not that, since it causes most trouble at idle/low rpm, runs fine at higher rpms. Three likely candidates: O2 sensor, Throttle Position Sensor, or inadequate fuel supply (could be either a plugged up fuel filter or a failing pump). If it is the O2 sensor, then it may be shorted- try a reading across the two wires (unplug it, check at the plug). From my files: Oxygen Sensor -
Located in the exhaust manifold. To test, using an ohmmeter, disconnect the O2 sensor connector. Connect the ohmmeter test leads to connector terminals A and B of the sensor connector. Resistance should be between 5 and 7 ohms. Replace the sensor is the ohmmeter displays and infinite reading. I don't know about you, but this test is pretty vague. They say it should be between 5 and 7 ohms, but infinite is bad. What if you get a reading of 13 ohms or 100 for that matter? I wasn't too fond of this test as I found it is a test of the O2 sensor heating element!. One other thing you can do with the O2 sensor is hook up the voltmeter to terminals B and C (I can't remember if those are the right letters, but you want to get the sensor wire (believe it was blue) and the ground wire, or just ground to something else) with the connector hooked up and the engine on, warm, and idling. You should notice the voltage jumping up and down in about 2 second or less intervals. If it doesn't jump up and down considerably or takes longer than 2 seconds to do so, change it.
If it is the TPS, then you should notice some weird (wrong time) shifts if you have an auto tranny, and you will have intermittent high idle (these are the symptoms on my 1990 with 4.0 engine/automatic--on third TPS since new). For the fuel pump/filter, you need to stick a gauge on the pressure valve at the front of the fuel rail, and see if it keeps the pressure up as you raise the rpms. You should have 30 to 40 psi- can't remember the exact number. If pressure drops at higher rpms, check for the fuel pressure regulator function (the small can at the front end of the fuel rail). Try clamping the rubber return line: if these leads to holding pressure at higher rpm, then the regulator is faulty. If this doesn't help, then the filter or pump are the cause of the lower pressure.
Also- electrical supply for the fuel pump: make sure that the white ballast resistor (on the fender in front of the air box) is well connected and not cracked. Those are the things I would look at first.
Most basic: how are the spark plugs/wires cap and rotor? I hope you have checked these!!
