Stalling when driving up a hill

I am having problems when it is going over the hill. It is very slow and if I have person in there sometimes they have to get out and walk. Whenever the it goes on the machine and all the codes are deleted it revs out okay. After a few hours it goes back to the problem. The machine is now showing that is is not transmission or coil issue it is saying it is an engine problem. It also has an oil leak from putting in a new transmission and changing the crank case. I need some help now. Thank you.