1997 Chevy Suburban rough running

I have a 1997 suburban 1500 4x4 with a 5.7 v8. A few monthes ago the engine started to start hard and run rough. After several trips the the garage and advice from some friends I have replaced several parts. It is better but still not running correctly. It did have a small fuel leak in a line so all the fuel lines from the tank to the engine were replaced. I put in a injector (spider) pump. The fuel pressure is about 60psi. This was the up graded convertion one. My voltage was a little low so I also replaced the alternator. It is now reading about 13 volts after for a while. The battery is only a few months old. The rough running starts at around 1400 rpm and is at its worst at 1700 rpm and goes away at 2100 rpm. If I keep it over 2100 it runs great. There is a little over 100000 miles on the truck and engine. Any help any one can give me would be great.