Power loss
 

Okay, here's a weird one. During the past couple of days, I've noticed the typical lower MPGs during warm up - no problem. Once that's done, I am getting great MPGs, typically about 52mpg at 55mph. Then, about 30-35 minutes into my 50 minute commute, I start to struggle to maintain speed / MPGs for a given LOD and fuel input.
Known issue: I've had a P420 code on it forever. After replacing the catalytic converter, the code remained. I think the O2 sensor after the catalytic is bad. This shouldn't affect FE or power.

Any thoughts?