close it up and wait a couple of days. If it's irrepairable, it won't harm it. If it's just a weird fermentation, then it's good to wait. To me, it almost smells like it's a sulphite problem, but since I've never experienced the rotten smell, I don't know with 100% certainty what to do.
But, this is what I would do:
Don't dump it quite yet.
Make sure the temp stays around 70-75 degrees.
If you have to look at it (and I do, so don't feel bad, hehe), stir it now and then. I know a lot of people say "don't! too much air time" but I don't think a 1-minute air exposure to my wines has ever damaged them beyond repair.
What's the SG on it now? If it's still dropping, that could be a good thing.
Keep us posted, and hopefully Glenvall can tell you more on the rotten-egg smell. He's much more knowlegdeable.