A week ago I began noticing jerking(It feels like a misfire.) while on the freeway.

I can accelerate fine but if I let off the gas once i get to 50mph it jerks sporadically until after 70mph after 70mph it no longer jerks.

Also, after a while stopped in drive(eg traffic/drive thru), I feel what I think is a shift in gears.

The check engine light is not on.

I checked the code here P0420 Error Code: Catalyst System Efficiency Below Threshold (Bank 1) a autozone employee told me it cant be the catalytic converter because its not old enough.

It a 2006 but has 126k on it. Any help on where to start is much appreciated.