Allison transmission prob

[Larry568]

I’m having some prob with a alliso md 3066 and I was hoping someone could help. It’s in a 2003 Ford sterling truck that I purchased as a salvage to rebuild. I finally got it all put together to find that I have some type of transmission prob . Going down the road after cold start it will shift up to 4th gear and it sticks there until I pull over and shut the key off once restarted it will start shifting but it shifts really fast and slips into 3rd. Has anyone else ever had this prob ? I have also reset the codes in it and it’s still doing same thing. I’m not even sure who I can take it to in my area