Automatic Transmission - [PLEASE HELP] shifting too soon and coming out of gear

When I take off mildly I end up in OD by 30mph and then I floor it and stays in overdrive. Also it comes in and out of OD while just maintaining speed. This problem happened before and after I rebuilt the trans. I thought it was the TPS, but I just put in a new one and it still happens. The problem started after I got the motor rebuilt with a cam and a pcmforless tune. It did sit around for quite a while stuff was broken. Before the rebuild very rarely it would have the shifting problem and now it's all the time.

bump please im lost. i want my car to run right. can plug wires causing a missfire cause problems. also i hooked it up to a cheepy scanner and it said slow O2 bank1 number 1 response.

or point me in a direction of who could help out. im out of ideas and don't have $200 to give to a mechanic for diagnosis.