90 4.0 xj w/hesitation & backfire

I have a 90 4.0 xj that has a really bad flat spot in the throttle. When you give it gas it hesitate,sometimes it will stall, it will also backfire a little through the throttle body. I cleaned the throttle body over the weekend and the iac. Did not make any difference. If the map sensor vacumn is unplugged, it runs perfect,but it smokes like crazy. I tried another map sensor, but this made no difference.
Any thoughts,ideas or guidance is appreciated.
Thanks
Scott

Does it happen at low RPM´s, higher RPM´s or all RPM´s? How is the idle?
Mine had a flat spot at around 2000 RPM, it would kind of thunk thunk thunk, in fourth gear, with an occasional pop through the TB. Also did it in 3rd but was much less noticable. Finally figured out (after looking at a whole bunch of other things) it was the 02 sensor leaning out the motor.
There are other components and sensors, that will cause pretty much the same condition. I had a TPS, that had a hitch at about 1/3 rd throttle, the motor would kind of stutter. Had a slight cross fire at mid to high RPM´s and had to reindex my distributor, guess the gears are a little loose. Some ignition problems make for a crappy idle and lower top end RPM´s (motor craps out around 3500 RPM). Missfires, are often more noticeable, with the motor under load, accelerating in higher gears. A partially stuck open, EGR is usually most noticeable at slow speeds or low RPM´s. You can look down the IAC opening and blip the throttle slowly and watch it function. If the IAC piston isn´t moving, something is probably wrong.
Just a few ideas.

My 87 does the same thing, cleaning the TB usually takes care of it though.

The idle is good,the problems start at around 1000-2000rpm, it will smooth out above 2000. Have not tried driving it to see how it acts on the road. I traded for a couple weeks ago, and I'm trying to de"bug" it.
Thanks

If it thunks and bucks at around 2000 RPM in fourth while mildly accelerating and the motor kind of craps out at around 3500 RPM. It could be the same problem as mine was, failing 02 sensor. I bought mine from the dealer, they couldn´t figure out the problem either and gave it to me cheap (I had to pay the previous owners bill, he told them to keep it).
But there are other things that can cause the same symptoms.

more than likley a tps or egr valve