C 6 locking up?

Trans runs great doing 1/4 mile pass. After I go thru the traps when I slow down to go get slip I leave it in 3rd. Once I get the
slip I shift down to 1st then try and leave trans locks up. I have to stop put in park then back into 1st in order to move the car.
I am thinking the convertor is lock up in the trans. Anyone have this problem before?

Could the shift linkage be a touch off? Years ago I once had an issue with my Ranchero where it would get hot and vapor lock, while driving down the road. I would shift it (column shift with cable linkage to trans) into "N" and attempt to start it while still moving. When I engaged the starter in trying to start it, it would lock up the transmission(left tire marks while going 40 or so). I finally figured out that the linkage was just off enough for the trans to act like it was in neutral until I attempted to start it, doing so made the trans go into reverse, even though it would coast when not trying to start it. Hope this makes sense. Todd

Makes a lo sense
I have had problems with the shift linkage before. I will go over it again.
Thank you.