blunty 07

i was wondering if any one can help with a problem i have with my car.i have put a 1.6 cvh xr3i efi engine in my mk3 escort.it starts and runs fine but when i stop the car, sometimes the engine drops the revs which then cuts out.i have tried the plugs,leads,throttle position switch,idle speed control valve and have replaced the petrol filter.it only does this every so often but not all the time.any help would be greatfull.Mike