I had similar problem with my 316 I discovered it was a problem with the brake load sensor under the van.
It is a metal spring loaded device that applies more pressure to the rear brakes when the vehicle is loaded.
Mine was stuck on and therefore the vehicle was very laboured as your effectively try to drive with the rear brakes on.
Next time you drive it for a couple of miles touch the rear wheels if they are red hot then maybe thats the problem?
good luck
craig haines
January 2008