335i Cab tyre probs

[kayc]

Ive got a 335i cab with run flat Bridgstones on it that have covered 4k miles in very normal driving...i noticed a slight wobble from the rear so had the car checked out by my local 'superhelpful' BMW dealer who has informed me both tyres are bulging on the sidewalls and therefore need replacing at 320 quid each.Is the combination of run flats,nineteen inch wheels and hard suspension an issue or have i just been unlucky..is this a common problem on these cars..any advice appreciated.