Back in the late 70s I worked in a Fuel Injection shop and ounce in a while a bearing goes bad or something in the IP Governor comes loose and causes similar problems. Most often it is the bearing that goes first.
Some times the Rollers on the IP Camfollower/Tappets go bad too.

I have even seen a case on a 6cyl Maxadine Mack Diesel Truck where the Camshaft in the IP had sheared in 2 parts but the Jagged break area allowed the Camshaft to turn (but part was out of time). The intermediate bearing (in the middle) kept the IP Camshaft in alignment and the were able to drive the truck to our shop.

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84 300D, 82 Volvo 244Gl Diesel