2007 Scion tC Hesitation and sputtering on acceleration

My 07 Scion tC sputters (most, but not EVERY time I take off). It has gotten worse over time. Now when you give it gas, you can feel and hear the car sputtering forward and see the RPMs slightly jump up and down. The only consistent code I keep getting is the P0420 code bank 1. If you it the gas harder than you usually do to accelerate in traffic, it jerks harder and faster, but also makes a flapping noise UNDER the car. The idle isn't the best either. Any information will help. Car is not under warranty, still being pod for, and I have limited funds to just keep replacing things it does not need. Thank you!