How often do you turn the RO system on? Sulfur smell isn't all that unusual - I've noticed a bad smell from my bathroom faucet lately when brushing my teeth. I'm not sure if it is the water or perhaps the drain. With the weather, my home shifting, the older pipes... sometimes it is hard to tell.
Burning off the first minute of RO water is usually the best way to make your DI resin last longer.