Strange problem, no code

Hello
I have a strange problem with my car. It is a -08 GT. When i start the car it starts off ok, then "short time fuel" starts to go down to -5% and then it compensates with "long time fuel" leading to a total of -24% on the fuel distributed.
I have replaced maf, fuel pressure unit and throttle body.

Since the fuel doesn´t go below -25% the car wont throw a code.

I have reset the PCM both with battery off and with the IDS (PCM replacement) but it still decreases the fuel.

I would be very grateful if anyone has any ideas about this problem.

Best regards from Sweden.

How can i elaborate when i dont have any more information?
If i knew what the problem was i would fix it.
I would really like to get some suggestion what can be wrong.

I will try to make the question as simple as possible:

My cars computer sends out an impulse to the engine to decrease the fuel supply. The car runs worse because of this (almost like an old engine with incorrect ignition). The car does not send out a code, because the amount of fuel decreased is not as much as needed for a code to be put out.

I have been thinking, and another question is if both cams are exactly in the same position,but the wrong one could this lead to my symptoms?

No modifications. I bougth the car from an insurance company.Afetr coming from the painter the car started acting up, nothing before that. I have had many mustangs and i am very familiar with them.

Any ideas what would happend if a temp sensor gave the wrong temperature. Would this cause my problem?

Since this happens on both banks, i need to find out what on the engine could control the fuel delivery and not throw a code and is not bank specific. Puh...