I have an 1987 4x4 ranger, 2.9 6cyl. 67K miles on 2nd engine, I bought it used for $450 & it runs fine until it is warmed up & then in any gear, exactly 2300 rpm's & up it runs like it is mis-firing, chugging, & loses power. I put on a new fuel filter, plugs,rotor & cap (old cap was really ozoned-up), checked all vacuume lines, put another "rail-mounted" fuel pump on, checked the coolant-temp sensor(ok -above the thermostat), checked the reading on the throttle control (with a voltmeter,6-9 & works) the EGR works good (cold has a good idle) I know this truck has another pump in the tank (sending-unit) . is this a "supplemental" pump for higher rpm's? this truck starts on a snap-of-the-finger & idles fine, does anyone have an idea? I am thinking it's something to do with the distributor/sensor (temp related?) or maybe that in-tank fuel pump. anyone??? I didnt see any other posts on this subject & i havent tried the "hammer the tank" method to check for a bad pump.
I only own Fords & have never dealt with something like this.

 

Did you change the plug wires also? Alot of times they will run fine at lower rpm's with bad plug wires, then at higher rpm's or under load start crossfiring when they get old and start breaking down. Had it happen on a couple vehicles of my own before.