3.5L DI engine, CEL question

I have this in my 2004 Rodeo, the only year Isuzu used their Trooper drive train in the Rodeo, so I'll ask this question here: Occasionally loose some power, most often about when the OD is trying to kick in, and frequently 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 3.5 L DI engine. Otherwise I'm stumped. Any suggestions?