1995 GMC C1500 Low power

Just bought this truck and it barely had enough power to roll down a hill, but it idled great and still does. If it was a "2" when we bought it, it's improved to a "6"now with the work listed below. Engine mechanical parts all sound good. Under hard acceleration it stumbles. You have to back out of throttle for it to pick it up speed. Have replaced plugs, wires, cap & rotor, and TPS. Cleaned and tested EGR. MP sensor tested good too. Checked timing with sensor unplugged and adjusted it to 0 degrees.

This thing runs good if you just let it get up to speed slowly. But when you can mash on it hard, it just coughs and goes no where. The only thing that seemed to help was adjusting the timing 10 degrees before TDC. The performance gets better, but it's still not powerful enough to pull a long hill or tow a boat. Any ideas please?