Poor idling and excessive "hunting" of my Ford Mondeo 2.5 si V6?

I have a 1998 Ford Mondeo 2.5 V6 si which when idling "hunts" a lot and then cuts off. This happens once the car is at normal running temperature but not when the engineis cold.
The first agent I took to said it was the O2 sensors - this was repaired at considerable cost. This did not help. Another said it was ECU - this was also repaired again at great expensive. The problem persists.
Please help