2001 chevy impala rebuilt Transmission Problem

I have just had a rebuilt transmission placed into my car.. This is the fourth time now and each time the shop cannot seem to fix it right. They are accredited through Aamco "if i spelled that wright I dont think so"
Well here is my problem this time.. The car shifts great but.. Say when the car is in second through fourth gear, if you wanted to hit the throttle pedal more than 30 percent of the way to accelerate the car will suddenly seem to pop into nuetral and the car will rev and slam back into gear. Or burn itself slowly back into the gear, but for that i have to let off the gas pedal when the car starts to slip into nuetral so that it can get back into gear. Its really bad if i am approaching a traffic light and it turns before I get a chance to stop because, the car then will shift into second like it should at about lets say 25 MPH reach 34 MPH then suddenly spit into nuetral reving the engine up. On the highway forget it! i dont even attempt that now because the car wont accelerate that quick off the rampway to reach highway speeds. Instead it will go nowhere. Please if anyone knows what could be happening please let me know. I dont know that much about automatics but i guess maybe the clutch or hydrolic pumps. But am not sure.