Reduced Engine Power alert 2004 Pontiac Grand Prix GXP Supercharged

I have this 2004 Pontiac Grand Prix GXP fully
loaded 3.8 Supercharge and I just bought it last
month (July). Well the first couple days of having it
were FANTASTIC! However, the guy who sold me
the car failed to tell me that it had an overheating
problem. No biggie right? Yeah that’s what I
thought, well come to figure out it ended up
needing a NEW RADIATOR, again NO BIGGIE. It’s a
PONTIAC! The engines last forever, so 207$ later
for a new radiator and the problem goes away for a
moment. Fast forward a couple days.. All of a
sudden the car starts telling me that “theft system
not programmed” so I ended up contacting the guy
who sold it to me and he had the nerve to tell me
(owner of this is my 4th Pontiac) that the car is not
equipped with an alarm system. Umm hello, I
maybe a girl but I’m not stupid by any means ALL
PONTIACS HAVE CHIPPED KEYS. So, I go along
with what he tells me (even though I know that’s not
correct) and lone and behold all of a sudden every
time I stick my key into the door to unlock it, and
open the door, the horn starts honking, lights start
flashing until I get my key in the ignition. THIS
HAPPENED EVERY SINGLE TIME ANY DOOR WAS
OPENED FROM THR INSIDE OR OUTSIDE. Well
after about 2 hours of that, my friend disabled the
horn so that at least it wasn’t drawing all kinds of
unnecessary attention. Fast forward 2 days, as I’m
dropping my friend off at the hospital here in
FLORIDA (where it’s 150° at 7am) so he can see his
new born a baby, I have my 1 year old in the
backseat thinking everything would be fine, BUT
WAIT, guess what??? As I go to start my car again
and leave my car shuts right off after I turn it over.
So at this point my car has disabled itself thinking
that it’s being stolen, in all reality it wasn’t. Now
Remember guys I have my 1 year old baby girl in
the backseat in her car seat in the heat. I ended up
calling my fiancé who came to save us and told him
what it was doing.Got on  the phone long story
short, the guy ended up giving us 500$ back to
take care of the radiator problem and the
inconvenience of the disabling of my car with the
alarm system. So FINALLY we get the car started
and head home. Over the course of the next 2
weeks everything started to go to hell. My check
tire pressure alert came on and then then the
GRAND FINALE! The Reduced Engine Power alert
came. Which it didn’t really make much of a
difference except for when you have a
supercharged engine when you hit the gas pedal it
should basically throw you back in your seat if hit
hard enough (trust me I tried it) but at this point it’s
kind of like it’s in limp mode. However, I could still
do over 65 he’ll I could hit 90 but just not as fast.
So my question is, What does Reduced Engine
Power mean? I don’t have any smoke coming out
my tail pipes, my radiator has been replaced again,
I don’t have any leaks on the ground that I can see
or smell. Everything I’ve looked up is telling me
throttle body sensor or throttle body kit. Any ideas
guys...

Sorry for the extensive explanation but that’s the
only way I could show you how and what is going
on. I do hope you enjoyed the story though! It’s
been a blast in my life.