Reduced engine power

Reduced engine power light keeps coming on in
my 2008 Grand Prix I can still do highway speeds
with it on however I can fill it retarding the timing
more and more as I'm going down the highway and
sometimes it doesn't want to shift when the
reduced engine power light is on when it's on if I
shut the car off take the key out and open the door
causing all the lights to shut off when I turn it back
on the reduced engine power will have gone away
and it will run right again but it always comes back
eventually it seems like it does it worse on hot
days I knew I needed a catalytic converter and
thought that might fix the problem I replaced that
yesterday and cleared all the codes it's still
happening it seems like it started to happen right
after I upgraded to a cold air intake which wasn't
really done right I sort of made one up myself do
you think if I put the intake back to normal it would
help? I'm confident I didn't forget any sensors or
anything like that when I did my intake