My first one to do the slow to 3rd was a backyard tranny that had laid around a long time. 50-55mph and finally shift. One day it made a whirr snap and that was it. The torque converter had spun on the shaft. After that it would only work til it got hot.

The last trans started doing the same thing after I had it apart and put new seals on the shift lever and new filter. I didn't know about torque values or anything so just tightended the valve body and called it good. On this one though I didn't drive 50 to make it shift and depending on temps outside I could go a half mile up to 5 miles or more befor e it shifted. It had to warm up and after that it shifted normally til it got cold again. I think it had a blockage so fluid had to get thin before it worked right.

If yours is truly a new rebuild, take it to the shop that did it and have them do the adjusting. The shop that did mine is a 1 yr or 10k miles (something like that) warranty. I'll hit 1 yr before I ever hit those miles!

If the kick down was adjusted too much that it depresses the lever it affects 1st 2nd shift I noticed with mine as I didn't have a tension spring on it to start with. So don't think the kickdown has any effect as far as your problem with 3rd.