2006 grand prix base 3.8l v6 not super charged...

My car has a hard start.. It has to be turned over twice like
it does not catch the first time. Also acts as if its not
getting the power it needs to start started! It also has
started this not catching, like ill push on the gas as if to
speed up or go faster and she wont go faster, then all the
sudden realises i hit the gas and goes...  Not a weird
whining like noise u hear only when she is warmed up and
the heat is on, and inside the car. And once in a while
she.will die and i have to restart her... I have changed the
radiater. Water pump. Spark plugs. Flushed the whole
radiater system... From the radiator cap... I have changed a
o2 sensor in the engine. I have perfect heat and perfect
a/c. I have done so much to this car. I have spent so much
money on all my hometown mechanics guessing what it
could be. And im bout over it. I need help. It hat noise
come from behind the dash. There is twice the heat went
out... There is a weird but not sweet its like a burning or
foul smell only once in a while. Everyone says if i keep
changing parts on it, its gonna be new...  Everything i have
changed was original to the car from the factory.