I have a '99 3.0 and runs really rough from 1500 to just about 1700 rpm. Doesn't matter if it's in first gear creeping up a hill or in 4th on the highway. Though in first it almost stalls.
It runs worse in the evening when the air temp drops and is down on power at all times compared to my 2003 3.0L.
There are no codes or check engine light.
I have replaced the plugs, wires, and coil. Air filter is clean. IAC and TPS have also been replaced (though not new. I got 4 of each from the junkyard and none made any difference at all)
The new plugs are now fouled from running rich.
Ideas? MAF is my next guess.