I have an 89 XJ, replaced many parts, ie. injectors w/ #19's, CPS,TPS, Fuel pump and tank, Tune-up w/ Champion Truck plugs, CAp and Rotor. New Cat back exhaust and O2 sensor.
THe problem lies when cold, put in gear, slightly accelerate, it will miss, pop, unless I mash the pedal, than it takes off. Once warm, it will usually accel from a stop fine, but once you reach a "cruising" speed it will begin to miss, pop, unless I begin to accelerate, than it runs strong! Today it was about 30 degrees and I let it warm up, checked resistance at the MAT sensor, read 1.2k ohms. WHen I drove it, again it would stumble as before, I then disconnected the MAT, and, the missing and popping stopped. Reconnected, miss and pop. I understand it has to do with adjusting the air/fuel ratio. SO it is now "thinking" that it is VERY cold, and causing the injector pulse to be longer, but it will take-off from a stop with NO Problem, not loading it down with fuel. Is there something else I should be looking at,that disconnecting the MAT is helping to compensate for?
Oh, all sensors and fuel pump are dealer parts.

Okay, here is an update. I have running it for bout a week now, fuel milage is average, and seems to run better the more I drive it. I packed 30's under it and it still has the power to turn them if I mash the gas. So I think I will leave it alone for now.

Here is what I found on the Mat sensor:

http://www.lunghd.com/Tech_Articles/Engine/Basic_Sensors_Diagnostics.htm

Sounds like the cold weather is causing incorrectly readings. When you disconnect the Mat sensor, the system is using a dummy safe mode reading.