Volkswagen Jetta 2.5

Hello guys.
I am having an issue with my VW Jetta that
is breaking my head and perhaps my
pocket. So if someone of you have the right
answer for me would be nice.

Everything started one day when by mistake
I did not tight the gas cap until the clip
sound after full up the tank. I drove doing
uber normally that day. At next day when I
turned on my car a light came out, that light
was the gas cap was loosen. So I went to
full up the tank and this time I tightened up
the gas tap properly. When I tried to start the
engine I didn't started and like to 3 tries later
it started but very low and then it stabilized
by itself. I started to work and around half
tank later around 125 miles and a 110° F the
check engine light came out and the car
started to do an annoying bad shifting.
When I was slowing down to a light and the
car start to shifting down I could see when
is going from 3 to 2 gear it does like hit, as a
problem with valve body. I took the car to an
Autozone and gave a code P0455 and
P0172 system to rich, and something about
vapor leak. I started to read about how to fix
the problem and I realized that may be the
canister vapor leak. Then I talked to a
mechanic and he told me that according
what I told him could be the valve body. But
when he checked the car told me that my
valve body is ok and he told me that my
problem and the codes were caused by the
air mass sensor flow. So I replaced the MAF
sensor and the mechanic with his computer
turned the light off, but the shifting problem
is still happen. Then I took the car to the
dealership and they told me that my
transmission is perfect. But I explained to
the expert about when the car has run 130
miles a day and it's hot start to do it and he
told me is the valve body.
But what it's confusing me is why when the
tank is in a half even in the morning have the
same issue, and when the tank is full the
transmission runs perfect. Right now after
full up the tank the engine starts great. The
only thing I have not done is when the car
and the weather is hot, the half tank, and
having the problem, go to full it up and see if
it still the shifting down problem.
Thank you for your ansewers and I keep my
hope it's a sensor and not a problem with
transmission.