po420 mystery

I own a 2008 Chrysler Sebring Touring Convertible with a 2.7 liter engine with
64000 kilometers on it. My problem is that I am getting a p0420 code
showing up every second day or so. I know it means that the catalytic
convertor is not working up to par. Here is what I have done to try and get it
fixed. I have taken it in three times and they say the catalytic convertor is
working properly after doing all kinds of tests on it. The car gets great gas
mileage and it has never dropped from when I purchased the car. There is
lots of power. I was told that it was the upper O2 sensor, so I had that
replaced. The code came back 2 days later. They changed the lower O2
sensor. The code came back. They put it back on the scanner and could not
find anything else wrong. They said to run seafoam through it and take it on
the highway for a long run. I did that twice. The code came back. Back on the
scanner and still nothing showed up but the p0420 code. They said to try and
change the gas that I used. I did that. The code came back. They said to buy
a scanner and remove the code whenever it came back, because they
cannot find anything else wrong with it. I did that and after every trip with the
car I plug in the scanner and reset the computer. Now for the interesting part,
whenever it is colder than -10 I can drive the car for weeks without the code
showing up. As soon as it gets to -9 the code comes back. I am at a loss on
what to do now so I hope someone can tell me what to do.