302 efi engine problem

Ok i own a 1995 mustang gt with a 302. my car will start up fine and hold idle fine. i can drive it for like 2 hours max then after i come to a stop my idle will begin to chug downward to the point the car dies. i am getting no trouble codes and here is what i have done so far to fix this...

i put a 1991 302 EFI from a crown victoria into my mustang(motor & fuel injection system), it was running exelent till I replace manifold with header. I drove the car one week without o2 sensors then one cold day i went to start the car and wouldnt idle when cold, and from then on when i push...