My 04 t6 was doing great, then I filled up with gas , a hour later when I started it it was shaking and there was loss of engine power when I drove off and a flashing cel then it would stay solid. It takes forever to get to 20mph. I tried several bottles of gas treatment, then I spent the next week driving around to get rid of the gas and filled up with some Shell VPower gasoline, got some power restored after going over 30mph but still no power just to get up to 20mph with gas pedal floored. Don't know what to do.

 

What codes is it showing?

 

Shows P303

 

That's a misfire on your #3 cylinder.
Check the spark plug.