stoves 900 gtf model 059041459?

this double oven and grill is now 12years old and has been of excellent service.
recently a fault has occured with the grill.
switch it on and it lites and everything appears to be normal but after about 10 minutes the gas goes out and the ignition activates trying to relite the gas, when the gas goes out there is a noise that can only be described as a deflating sound. switch off for a couple of seconds then back on and the grill lites as normal. this problem keeps repeating itself as you use the grill.

thanks
regards
dave