Hi there,

I am not a mercedes technician, but would bet, that you have one injector "dripping". i.e. not closing all the way.
If your fuel distributor was gummed up from sitting, a gummed up injector would not surprise me.
The reason why the engine runs better with a larger than normal gap is, you are getting a bigger ( stronger ) spark. Hope this helps you a little.
P.S. If it was my car, I would try a triple dose of fuel injector cleaner first.