Sputter Putt Putt Sputter

[toneloc7777]

I live in Thunder Bay, Ontario Canada in a city of about 110,000 people at the top of Lake Superior. Needles to say its pretty tough to get any Mercedes advice here with only one Tech at our Local Dodge dealer.
Anyways, I have a 1997 E320 Sedan, Charcoal grey with a 3.2 gas job.
When I first bought it it purred like a kitten no matter what. I drove it throughout the winter and just recently I started to have sputtering problems with it. When it is in drive or reverse it starts to sputter like its starving for gas, but as soon as I throw it into neutral it hums. It has gotten a bit worse over the last couple months.
Does anybody out there have any Ideas what it could be????
Thanks My email address is tony.p.444@hotmail.com