Van shakes going over fifty mph, code P0420

My minivan shakes when on the freeway going over fifty mph. If I take foot off the accelerator the shaking/vibration stops and engine runs smoothly. The car has not had a tune up anytime recent. Regular oil changes done at four to five thousand miles. Check engine light is on with code P0420 - Catalyst system efficiency below threshold Bank1.

It first started during my return drive from Miami to Chicago with family about twelve hours into the trip. I was cruising around eighty mph and started hearing brief intermittent vibration, about one second every few minutes. The vibration sounded like driving over a rumble strip. Called my mechanic and he suspected cylinder misfiring. After a while it started becoming more regular. No vibration when foot off the gas pedal. I took an exit to check and noticed that the vibration is not happening with lower speeds. I got back to expressway and drove around fifty mph without any vibration. Over the next few hours I had to bring the speed to around forty eight mph to avoid vibration and got home four hours late.

The next day, took the car to mechanic and he suggested wheel balancing since vibration was happening above fifty mph. The problem appeared fixed as I took to expressway and drove about twenty miles at different speeds fifty to eighty mph. The check engine light was reset by mechanic next day after an oil change.

Three days later I drove the car about fifty miles on expressway in the morning and on the return trip (about twenty five miles in), the vibration started happening again and I was driving around seventy mph. Once it started vibration, I tried driving at various speeds and it was happening at all speeds above fifty mph and consistent. As before, vibration stops when gas pedal is released. After a while check engine light came on with same code P0420.

I have not noticed any major loss of engine power. I have driven the car on short trips (under ten miles) on local roads around forty five mph without any vibrations in last couple days.

Wondering whether to replace Bank 1 catalytic converter. Any suggestions? Thanks.

Also for some history; had no major problems with the van in the past thirteen years. Bought new January 2005; Timing belt done at 105,000 miles, engine mount replaced around 120,000 miles, front passenger wheel bearing replaced a month ago.