Ford escort cutting on deaccelaration

I have a 1993 -1994 ford escort 1.8 16v.Once the car has warmed up and driving at speed once you come to a stop in most cases the engine rev's drop below normal rpm to very low rpm and tends to stall or struggles to keep goingfor a few seconds then returns to normal rpm this also happens if you suddenly come off the gasand try to stop the car quickly ie ( emergency stop)
Can anyone help?. Any ideas regarding the fault ??