Rebuilt automatic transmission already slipping 6 months after install?

Question

The car is a 2001 Ford Mustang V6 RWD. It slips in 1st gear but catches itself but shifts hard. Same with reverse gear. All other gears are perfectly fine. Fluid and level are good too. I don’t understand how this happened, can anyone please help? :(

Update:

The car was running great since I got the new tranny installed until 2 days ago. It only slips when it gets warmed up after 15-20 min of driving.

Update 2:

The only thing I can think of left to do is a transmission fluid and filter change, and putting some Lucas Transmission Stop Slip and seeing if it helps. The tranny has about 5k-8k miles on it

Answer 1

Take it back to where the transmission work was done. 12 month and/or 12,000 miles is the usual warranty period on a replacement transmission.

Answer 2

Dumping an additive into it may void any warranty you have from the shop.

I've had many Mustangs . . . . I stopped buying them when they starting using automatics made in France (well known for not making anything right).  The ones I had also had a hard shift from 1-2 . . . other shifts acceptable.  I took the last coule cars back to the dealer when virtually new and was told that there was nothing wrong with them.  I buy Hondas and Hyundais now - Thanks for nothing, Ford.

Answer 3

Well take it back to the place that rebuilt it for you. Chances are it would just be a simple adjustment.