1996 Sable stumbles under load

I have a 1996 Mercury Sable wagon with 169K mi / 3.0L OHV engine. Recently the car has stumbled under load, especially when climbing a hill but also on level roads. The car lost power and the transmission came out of gear. The engine revved up so I pulled over and the transmission reengaged. I accelerated and everything was fine for a couple of miles then stumbled a little more but got me home. This has happened in the past. The ignition wires and plugs were replaced and that seemed to fix it. Those parts have only about 500 miles use. I also have a recurring P0420 DTC. No other codes present. I've shut the MIL off but it returns soon. Could my convertor be plugged and be causing this stumble? Could it be a worn 02 sensor? A clogged injector or the transmission? Perhaps a failing fuel pump? Thanks.