trans problems after rebuild?

[Daniel Jost]

Early this year i had my gn out after rebuilding the motor, i went to race my bros modded 05' duramax and at the launch i shattered the gears for first gear. After that the trans was rebuilt with the good internals from my brothers old 88' monte carlo ss 200-r4 and it worked good, it was like it had a huge stall if i was doing about 30MPH in 1st gear and let off it would go to idle, and i only have a 2800 stall, it never did this before the trans broke,and one day i did a boost launch and it seemed to pop out of gear like it was in neutral then it would make a fairly loud clunk back in to gear. Then one night something went wrong and at a stop sign leaving lighly it did the same kick into nuetral thing, but it never went back into gear no forward gears worked but reverse still did, well now im very good at backing up. after it was fixed this time some bad stuff was found dont remember what, my uncle a proffesional trans builder found some bad stuff and replaced it. now when i take it out it stalls the same in first, doesnt in second or third, but will stall and do the same wierd stall thing as first gear, but now it is not locking up or going into third or something until i do over 90MPH, can someone help me this is very frustrating. Thanks Dan