2004 Rodeo with 3.5L DI--CEL question

Occasionally lose some power, most often about when the OD is trying to kick in, and ofetn when going up hill. Flashing CEL reads P0303. I replaced the plug and coil on #3, and cleaned the wire harness to coil plug connector with contact cleaner. Still get the P0303 code.

Began to wonder if a bad Fuel Injector seal could be the culprit--it's the DI engine. Otherwise I'm stumped. Any suggestions?