What's wrong with my Mustang?

So, I have a 2004 Ford Mustang V6, and a few days ago, as I was driving it, it started acting up out of nowhere. It idles rough, it accelerates slow, downshifts and revs high going up the smallest hills, and it holds gears and revs high before jerking you into the next gear. The check engine light was on and I took it to the mechanic and the only codes he got were that one of the spark plugs were misfiring. So, I had him change all of them plus the wires and figured all of the problems were stemming from that. I picked up the car, and the light was off at first, but I started driving it and nothing had changed. If anyone has had these issues or know what they could be caused by, I would greatly appreciate the insight.

Rnaybe you should have a rnechanic check it out

Mileage Matters and you didn't say. If the car has over 100K it's good the plugs and wires were changed. The Injectors should be listened to, steady clicking They will cause Misfire codes too. And the Coil should be checked too, pull the Fuel Pump Relay and crank the engine to check the color and sound of the spark. Blue with a White core and a crisp crackle sound is what you want.

Take your car back to the tech. He needs to diagnose the fault properly before throwing your money at the problem, and retest the car before handing it back to you.