Car struggle when acceleratio

I have Mercury Sable 24V-DOH(2001)new spark plug wires and plugs were put in a week ago, and in the last two full tanks of gasolne I put some injector cleaner. A scanner showed P0306. I removed plugs 5 and 6 check the gapping and the wire connections. A reversal of the plugs and another scanning showed no misfiring. But on acceleration it still struggles. Could this be that the #6 injector needs changing or more cleaning?

Note two 02sensors were put in a month ago