99 GT rough lugging at medium acceleration

My 99 GT with 3.8L lugs very rough at medium acceleration, but is fine at low or high acceleration. Tach jumps up and down some during this jumpy lugging motion. Have changed:
- Fuel & air filters
- plugs and wires
- PCV valve
- local shop changed an oxygen sensor & cleaned mass airflow sensor
- local shop cleaned fuel injectors

In an attempt to see things for myself, I borrowed an ODB-2 cable & connected to my laptop. During lugging and while holding the accelerator steady, the TP (absolute throttle position) and MAP (intake manifold absolute pressure) seem to hold steady and are NOT jumping around.
Thx for any help. I'm hoping to get to the problem soon and quit spending more money.

possible bad gas? water in the gas?

Thx Rubberman...This lugging issue has been happening for a month or more, so I've used several tanks of gas from at least 3 different stations.

What brand of plugs and wires did you install? Any CEL and if so, what code(s) are you getting?

Thx for the reply richtazz.
Plugs are AC Delco.
Wires are an OEM brand - Global Solutions (lifetime warranty).
There are no error codes showing up, so whatever is happening, it must be happening within tolerances.

By the way, the lugging problem started before I changed all of the components.

The way the car lugs forward and back with the tach running up and down, it seems like it has to be something in a control system....i.e., a sensor or something that would cause it to cycle up and down in such a way.

I hope to borrow the ODB-2 cable again this weekend and do more checking...if I can figure out more of the codes and software...this is all new to me, but I do have a computer background.
Thx again for any help.

Did you ever get this problem resolved?

When you get the scanner post your long term fuel trim numbers. Possable vacuum leak? Does it seem as though its a missfire? rhythmic lugging? Can you duplicate it in drive with your foot on the brake?

oops sorry guys, resurected an old post without realizing it. I apologize, it wont happen again.

oops sorry guys, resurected an old post without realizing it. I apologize, it wont happen again.

No you didn't it was wkbl52 so don't worry about it. The thread is not that old to be nitpicky about it so we'll let it go. Also, has no resolution or update to the problem.

I did not get the problem resolved as I had to spend several weeks in Taiwan and China for work, and have since sold the car with the problem.

The buyer was willing to pay my asking price, even knowing about the problem.