My 87 Ford Ranger has a continual issue with stumbling, bucking, power loss while driving at any speed, when its cold it doesn't seem to do it, although at startup there seems to be a rough and erratic idle most of the time and trying to accelerate doesn't work when its cold. After it warms up during driving trying to accelerate will oftentimes result in the bucking, power loss, acting like it isn't getting any fuel. I'm leaning towards a Throttle Position Sensor (TPS), or the computer (EEC).
I just replaced a TPS on my 99 Jeep with the 4.0L 6. It would stumble at idle and when accelerating until it warmed up. I had replaced the TPS previously so didn't suspect it. I replace the part again because it set the check engine light when I gave it a little more throttle than usual. The code was that the sensor was outside of its expected range. It's my fault because I used a 15 buck TPS from eBay. I like to buy cheap parts on eBay. In the case of a TPS, that's probably inadvisable.
I got one at the NAPA down the street. The XJ works just sweet now.