Overheating while idling. Slipping transmission when it gets hot.
Twenty year old car. I suspect a lot of preventative maintenance was delayed or not performed.
Make sure the engine compartment fans are turning on, and at the correct temperature. In meantime, monitor the coolant temp gauge carefully. You definitely don’t want to allow the engine to seriously overheat. Sometimes turning the heater to MAX will provide add’l engine cooling, as a temporary work-a-round.
Asking for a diagnosis from a local well-recommended transmission specialist is your best bet. I expect this is worn clutches, & will eventually require the transmission be rebuilt. Ask your trans expert if replacing the transmission fluid might buy you some time.