I have 200# on you, and I easilly see 85mph on highways. of course most don't belive me on the speed of my 126 car. if you are having problmes getting up to speed or maintaining speed, there is a problem with your airfilter, turbo, or alda fuel boost valve.
fuel filter can also cause the problems you describe.
go to dieselgiant.com and look at the mercedes repair sections good place to start.
John