i once had somthing similar on my '81 200
however i dont think it could be the same thing, cause mine is a carburated petrol engine and yours is, i presume, a direct-injected diesel?

anyway in my case it was a rubber diaphram in the carb. so every time i pressed the throttle, it wasnt actually working, thus max. revs were about 2,000 as opposed to the normal 6,000

..

could it be dirty injectors or something?
or.. a wild guess here.. a really, really clogged air filter? :Confused: