It almost sounds like a brake problem could be causing the power loss. Perhaps a caliper is sticking making it hard to accelerate. I can't imagine that the brakes could self apply as you were going along. When you stopped along the road did you smell overheated brake pads? An overheating brake can heat soak and boil the fluid making braking poor until the air cools the fluid in that brake and it can condense again. I think I would jack each wheel up and turn each wheel looking for abnormal drag. Wheels with drum brakes will turn easily and disk brakes will drag on the pads but will be fairly easy to turn by hand. Of course a stuck or seized caliper would only apply to disk brakes.

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'97 E 300 D