I have installed a 04 cobra engine into a 56 f-100 with the original harness and ecm. The ecm was sent out to delete the PATS and emissions equipment as well as program for return style fuel system to get rid of the pump drive module. The engine will start and run, but the rpm will drop 500-600 rpm then recover every 4-6 seconds and the a/c compressor and low speed fan will will cycle on/off at the same interval even though the wiring to both low and high pressure sensors are not hooked up. Even holding the throttle to 3-4000 rpm still makes it happen. The only thing that keeps the rpm from dropping is constant movement of the throttle, and its injector pulse that is being pulled to make the rpm change. I have tried 2 different ecm's from 2 different programing companies with the same results. Every input and output to the ecm has been checked to make sure nothing is grounded, shorted, or open, and all sensors are reading correctly on the scan tool. I Know originally there were 2 ecu's that communicated through bi-directional data to the ECM, the instument cluster and the ABS module, but have never had lack of communication to one of the cause running problems in any other vehicle before (without PATS). Has anyone had this issue before? Any ideas will help, thank you.