Ours did the same,2 hours after cooking,ours would be 4 years old in the flat we rent,I had a new one laser cut from brushed stainless,cleaned the old glass & sealant off & refitted with 300deg resistant sealant.
I say it's a design fault
,a combination of stainless & glass is conducive to eventually the glass breaking down due to the conductivity the metal.
Our Landlord pleased as I sent pic of job to pass to the owner,this needs investigating very dangerous.
GEOFF
April 2013