The main clues in the story are that your breather hose blew off recently and you were going downhill.
Here's my hypothesis: You built up pressure in the crankcase which caused the shut off valve to engage, turning off the engine. Pressure in the crankcase is roughly equivalent to vacuum on the other side of the shut off. Get enough pressure (by plugging the breather hose or in your case, something else) and the engine will shut down.
Something is causing high crankcase pressure. Excessive blowby? Hole in a piston?
The reason it happened going downhill is that the engine was not combusting, and burning the crankcase gases, allowing them to build up.
To test my hypothesis, T into the breather hose with a flexible hose and attach a pressure gauge and run it back into the passenger compartment and monitor your crankcase pressure.
Redneck solution: T into the breather hose with a valve and a hose running to atmosphere. Control the valve with a cable, like a heater valve is controlled. When going downhill, open the valve, temporarily venting the crankcase to atmosphere. Close the valve at the bottom of the hill.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13