Why does my 1993 Explorer lose RPM's while driving?

1993 Ford Explorer 4.0L 2WD While Driving The truck loses RPM's cold or at operating temp. I can give throttle and the RPM's will climb to 2500 or so and drop back down to 1000. If I engage the clutch, turn the ignition off and back on, release the clutch, all is fine. At this time I can rev the throttle and the RPM's will hold at red line. I cleaned the MAF sensor with MAF cleaner and the IAC with brake cleaner. The truck ran fine for 40 to 50 miles then went back to the original problem of losing RPM's. I have replaced the fuel pump & screen (tested at 30 Pounds before replacement but only 29 pounds after ( Auto Zone said it needed to be 35 to 45 Pounds)) Also replaced the fuel filter, Throttle pos. Sensor and MAF sensor. There is apparently no EGR valve on this engine as it should be located near the IAC on the drivers side and it is not. I have read in forums that it might be a weak ignition coil. I am getting tired of the process of elimination game and am hoping you guys will have run into this problem before. Thank you in advance for any information that may help.