low power at light to mid throttle

I have a 98 gmc Sonoma with the 4.3 VIN w. It has 100000miles and is and auto. My problem is it has very poor performance at times. It accelerates slowly from stop to about 40 on light to mid throttle if you floor it it runs fine. It seems to run first gear out longer before shifting like if you were towing. Some times it will run great and other times it just falls on its face. Here is a list of what I have done. no codes

Complete tune up including
Plugs-ACDelco
Wires-ACDelco
Cap-ACDelco
Rotor-ACDelco
Coil-ACDelco old one was bad
Fuel filter-ACDelco
Maf sensor

It has 100000miles and is and auto

posting duplicate threads is not allowed. i will keep your other thread open since you have replies there.