Rebuilt tranny not quite right???

[beefyberry]

I have a 97 F350 powerstroke with an automatic transmission. My transmission went out, but a good friend of mine, who just graduated auto tech school, had a tranny out of a 93 7.3 pickup that he rebuilt. We put it in but it has a few issues. The first two miles it shifted perfectly, then on the way back it had a long delay into second and had a hard time getting it into o/d. checked the fluid level and all the plugs and tried again. This time it ran perfect again. I drove it for two days with no problems. When I drive it for a long time then it wont go into o/d at all and second delays. If I let it sit for a couple hours it drives perfect again. I know it will probably have to be gone through again but if anyone has any idea what could cause these issues I would appriciate your help. thanks