Vehicle has no power

I have a 1995 GMC 1500 regular cab truck with 4.3 liter throttle body Six cylinder. It has 120,000 miles.
It has been coughing while driving for some time. ALmost feels like it dies, although engine never quits. I put new plugs, wires and cap on. I couldn't get the old rotor off, but it looks good. The truck starts fine, revs good and runs smooth with no back firing. While test driving, it lacks power, it will barly get going. It won't build up power. After getting it back home, I checked the plugs and made sure wires were on good. I changed plug wires one at a time when I did the change. When I checked under the hood, I noticed that the drivers side manifold was hot, and couldn't touch it, but the passengers side was only warm and I could touch it.