Not enough power under load

Not enough info to be able to determine cause... But if engine has guts with light load then your problem could be related to the tranny or T-converter going down, i.e. worn out seals and or clutches, turbine not pushing oil as it was intended to... ask in google for symptoms of bad clutches or seals so you can figure for yourself before tearing down parts. >>> If engine has no problems then it could be what follows it<<<