Braking

Hi can anyone help me with this problem .I have an Audi A4 convertible on a 53 it's low mileage and seems to have been cared for judging by the receipts .I have got this one problem and when I'm coming to lights or stopping I feel like the car will stall but it's auto it doesn't do it every time ,when it does the the Rev counter jumps up slightly and then settles down .I don't have any lights on the dash and the car shows no faults under diagnosis .Im going to assume it's mechanical not electrical but I'm hoping someone on here may of heard of this or had it themselves.thanks