99 suburban stumbles

[nflskinfan]

Haven't been in for a while, but I looked to see if anyone has had my problem and couldn't find one. About a year ago, I had a rebuilt engine installed and the truck ran fine for a couple of months, then the fuel pump went out. About two weeks later, the transmission would only shift into the two lower gears. Had that rebuilt and after about 300 miles the truck would stumble as if it was trying to go but wouldn't. The only way I could make it make any power was to manually shift it and the tranny went out again. Got it rebuilt again and it seemed fine for the next couple hundred miles and the engine seemed to miss just a little at cruising speed eventually getting worse and worse as the days passed. The SES light has never come on while the stumbling is taking place. The truck sat for 8 months and when it was driven again, it ran fine, like a champ, for 400 miles. Now when the gas pedal is pushed, I can barely push the gas until I am over 25 MPH or it will stall, not cut off, and then surge. Thought it was a clogged cat, but that isn't it. Put new fuel filter in and it helped a little, but still no joy. Haven't ran faster than about 40 so don't know what happens at higher speeds, but taking off from a light I can barely push the gas until at least the first shift of the tranny, about 12 MPH and then a little harder until the second shift, about 22 MPH. There are no codes, checked with a scanner and no SES light. I use Lucas injector cleaner about every other tank full and I have 120K on the clock. Hope someone can help before I just decide that I don't want a money pit anymore and give up. Thanks.