it sound like the transmission is starting out in 2nd gear. if you have low take off power or if it takes a long time to build up speed that is probobly what is wrong. however if it starts off normal but does not reach top speed it is probobly stuck in 2nd or 3rd gear. does it shift 3 times then a softer shift when the converter locks in?
the transmission does not have anything to bo with how much hoursepower the engine puts out, it just converts your engines rpm's into 5 diferent gear ratios. my guess is that it is not in the proper gear at the proper time. I would suggest that you take the car to the placed that rebuilt your transmission and have a tech test drive it. you should have a waranty. thier is no "quick fix" for a problom like this.
I hope this info has helped you