The only 3 things I can think of that would cause that:
- Using green wood (wood that's not been seasoned long enough)
- Using some kind of wood other than hard wood such as pine, spruce, fir, etc.
- Improper airflow into and out of the smoker
Make sure the wood you are using is well seasoned, it has been in the dry for at least 4-6 months. I don't know where you are getting your wood chunks, chips from but make sure it is a hard wood and not a soft wood such as pine, spruce, etc.. The rule of thumb is that if it bears a fruit or nut and is a hardwood, it is normally good for smoking wood even if it is of the ornamental variety.
Make sure there is ample venting for air to get into the smoker down low by the flame and smoke box and up top above the top rack. On my GOSM, I have 2 vents at the bottom (one on each side) and then 1 vent on the very top.
I keep the bottom vents open at least 1/3 of the way at all times and the top vent is open about 1/4 at all times. Mine also has tab stops on the vents so I can't close it all the way even if I wanted to but yours may not have that or maybe they've gotten bent over so that they no longer work. Just something to check.
Q: Are you using the original smoke box and water pan that came with the smoker?
Make sure there is ample room for the smoke to get out of the smoke box and flow around the water pan, up to the meat and then out the top vent and that there is nothing that might be blocking that path.
Q: Can you think of anything else that may have changed, no matter how insignificant it might seem, since the time it was working properly until the time it started causing black residue on everything.