Rotten egg smell

There's been several times the car smells like rotten eggs. It used to be only in the outside of the car, but now it's migrated into the car. We have spoken to the service writer at the local Mazda garage. He said he's heard of a few complaints and try a higher octane/ different gas station. The smell persists and seems to be getting stronger. Any suggestions short of selling the car? Thanks for your time. Kim and Nancy Lake