03 Supercharged cuts out at 4000 rpm

I have a 2003 Bonneville with 3.8 Supercharger. The car has recently been getting a P0420 code - Catalyst Efficiency below threshold.

Also, the motor cuts out at 4000 rpm whether under load or not. I believe the code comes when the car is under load.

I have replaced the Fuel Filter, Wires, and Spark Plugs. Also I found and fixed a vacuum leak. She has 170,000 miles but does not burn any oil. Starts and idles like a new car. Gas mileage use to be around 28 highway but has dropped off to 23 over the las tfew years.

Is the convertor in bad shape or am I missing something?

Any help with this would be greatly appreciated!

:disappoin -Mike-

Update.
I unbolted the convertor and it looked clean. Fired her up and is doing the same thing (cuts out at 4000 rpm and will not rev any higher) with exception it sounded like a dragster this time.

Throttle Position Sensor?
MAF?

Any Ideas?

Update.

$150 and problem solved!

I installed a rebuilt MAF Sensor from AAP and she runs like new.