Idle Problem

I have a 2000 mustang GT.... just bought the car and rebuilt the motor. The motor is all stock except for the Cold Air Intake (K&N air filter) The idle problem was there before the rebuild. I have checked the following, all vacum hoses, sprayed carb cleaner all around did not find any leaks, the car has no check engine light or codes.I have checked the tbs and ohemd it out (fine), checked the idle control valve, ohmed it out& cleaned it and it seems to be working fine. I can hear it clicking when i turn the key on. The mass air flow sensor I thought could be bad so I had one from a 2001 mustang and it ran worse so, put the other back on.But if you unplug it (mass air flow sensor) it idles fine.I could use any help, I have a lot of time and money in this car

It will idle good for a few then drop off to almost a stall (fluctuating) Its worse when you first start it until it worms up and then its not to bad but still noticableThis is with the ac off.Yes cleaned the maf no difference.Like I said if you unplug the mass air flow it runs fine.Also I have checked the fuel pressure and changed fuel filter and its all fine.Put new moto craft plugs in it.