I have a bolt on ls1 with fti 3600 converter just recently when I reverse car slightly moves then stops and I have to lean on converter till about 2300rpm to get the car back moving trans shop says fluid and level is fine. And also I have 3rd to 4th slip and when the converter locks it comes right back unlocked.

 

Can't help ya with all of that but for the converter coming unlocked I had that problem. The reason it was happening to me was because the computer was detecting missfires in that range and was kicking me out. With the tune adjusted all is well.

 

OK thanks for your feedback I'll try the retune to see if that helps.