Hello Bronze, welcome to the forum, first do you have CO detectors and have they shown any CO?
Take a look at your coal, are there light colored stripes or blotches on the coal?? If so could be some bad coal.
You may want to try a few bags of coal from another source to see if the smell goes away.
Otherwise: when something changes: look for changes. Have you improved the windows or doors in your house making it more airtight?
Any new partitions, enclosed areas around the stove? Anything that may restrict air to the stove so it doesn't draw as well and is letting fumes into the house??
Have you added bathroom or kitchen exhaust fans? A powered vent hot water heater?? Anything else that may exhaust to the outside??
Do you have any horizontal runs in your chimney pipe that may not have been cleaned out and have an accumulation of fly-ash blocking the pipe?
Check the gaskets on your doors, and make sure they are in good shape. Check the chimney pipes, they may have rusted badly over the years and have pinholes letting fumes into the house. Regular steel flue pipe will corrode through in 2-4 years unless they are 'summerized' each season.
That's all I can think of right now. When is the sulphur smell noticed?? After you have been away from the house and the stove was turned down to keep the fire low?? or in the middle of the night with the fire 'crankin'??
Hope this helps Greg L