My 850 10v saloon only has 57,000 miles on the clock and is great exept for one problem. It imtermitently won't idle properly.

Symptoms are like this I'll start it up, all is fine, go half mile down the road come to slow down at a junction and when I deslerate to just over 1000 rmp the engine will increase its revs to approximately 1700 rmp. I will have to brake quite hard to get the engine to reduce its revs. When I do come to a standstill the car will be idling at just over 1000 rpm.

If I put the auto box into to neutral the revs will yo yo between 1000rpm and 1700 rpm. If I dab very severly on the accelorator this sometimes cures the symptoms but not always. Opening the bonnet and shutting it again sometimes cures it.

My regular garage have checked hoses and valves for leaks etc and can find anything wrong and say I need to take it into a mainstream dealership for diagnostics. This is a problem as I can never predicts when the fault is going to occur and I have experienced problems with my local dealership.

Has anyone any ideas what might be causing the problem?