2002 Legacy GT; Manual Transmission; 234K miles.
Here's the problem history:
1. Car was missing and skipping in high gears.; if I was in 5th gear and stepped on the gas while driving at 65 MPH on the highway, it would lurch, skip, miss, etc.
2. While I had this problem I also noted that my gas mileage had dropped from ~28 MPG to ~25 MPG.
2. I changed spark plugs and ignition cables - Problem Solved for the missing and skipping, etc. Car is running great.
3. But my Gas Mleage is still down. I filled up this morning and it's ~ 25.5 MPG.
4. I have an intermittent CEL (P0420 - Catalytic Converter); but I've had that for about 2 years now and my gas mileage has been consistently around 28 until a month ago. I have no CEL right now.

Any suggestions would be much appreciated.