Mk2 focus Revs drop

[Fordfootforward]

Hi everyone, 

I have a focus mk2 57 plate... when I rev it in neutral the revs raise nice and smoothly but when I release the accelerator the revs fall but drop below the idling point make the car sound like it’s going to stall, then it automatically revs itself back to normal idling rpm.

 

anyone have any ideas what might be causing this? 

 

Thank you you for having a read 👍