This is a nightmare

I have a 2006 pontiac torrent and I keep having
acceleration problems the mechanic keeps saying
catalytic converter but I've replaced it 4 times with
the same problem returning the check engine light
codes says it's running lean ( P0171) with catalytic
converter codes as well it will be ok for a few days
then start doing it again...when I'm driving it will go
into a coast not losing power I'll push on gas car
then eventually will accelerate again but initially
with a big surge of gas causing rpm to go 5 then
be normal a few more miles and will repeat
problem and the problem is getting worse. The
dealer (Mini Cooper Autonation) sent it to
Buick/Gmc to have a full diagnoses and the result
was the same a bad catalytic .... As an added note
the dealer just put in a remanufactured engine in
which the light was on after picking up the car after
that was done .....updated info after this past
info:So I replaced the catalytic convertor  and Mini
Coop then reffered the SUV to Buick for a
diagnostic ...3 times because Mini did not agree
with the diagnoses that it was indeed fuel pump ...
It was to be sure (they said) Buick Gmc said yes for
sure....well Mini coop told them to do the repairs....
I picked up my Pontiac and it did the same
hesitation thing ( no backfire just hesitation) while
driving on the highway .... However no check
engine light (lean code) came on .....I returned to
the dealer (Buick Gmc) they then called me next
day and said the ground 105 was loose on the
newly installed fuel pump and to return next day to
get my truck..... I drove 1.6 miles from dealership
my check engine light came on I immediately
returned to dealership (Gmc) they ran the
comptuer check for code it was the lean code yet
once again.. I refused to drive the car until
something was figured out they gave me a loaner
car... That next day they called me and said the
problem was the mass air flow... And they cleaned
it out reset codes and it was good ...to come for
my truck ... On the way home my check engine
light yet again came on.. I keep telling everyone
that I never had a problem with check engine light
with lean codes coming on until new
remanufactured engine was put in... I havent been
able to tell if hesitation has been fixed while driving
on the highway yet because of me not being able
to drive it longer than a day. I brought out this
discussion to the mechanics at the dealership ....
One said we already know it's running rich not lean
or it wouldn't keep torching the insides of the
catalytic convertor.... Extremely frustrated I said
well no one seems to know what in the heck  is
wrong after seeing two dealership shops and two
other shops (meinke and a mechanic at a local
repair shop with high reviews on yelp) I have to
much negative equity to trade in and it has been a
total nightmare since I bought it...I still owe to
much on it to trade in