Newbie needing help troubleshooting p0420 misfiring

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.

Do a search in the upper right corner for that code P0420. Likely it may just be the upstream O2 sensor causing it, also make sure there is no oil leaking into the valve cover valley where the plugs are. How did the plugs look? This code usually means the CAT is going bad, but many people have replaced the first O2 sensor to make it go away. A search will provide lots of clues that may help.

[QUOTE=06T200;195802make sure there is no oil leaking into the valve cover valley where the plugs are. How did the plugs look?

Definitely, remove the plastic cover and look into the spark plug wells, could be oil pooling around them.