so its an auto but it might be the clutch slipping???
i think you mean the internal clutch packs in the trans.
did you check the trans oil with the car running and a warm trans??? cause if you checked it with the motor off, that means nothing.
older trans could mean slippage, which basically means the trans is cactus. ensure correct oil level, and if still doing it then id be heading toward trans internals.
probably need rebuild or changeover trans....