I went out to start my car this morning. No go. Engine fired but wouldn't stay started. The car started and ran fine a couple days earlier, with no noticeable lack of power or anything like that. I did finally manage to get it started hours later but not without a lot of cranking. I loosened the injector line nuts and fuel came out from all four, so I don't think it was a fuel delivery issue. Both fuel filters and the strainer were replaced/cleaned a couple weeks ago when I had the tank off to patch a leak. No bubbles in the line from the pump to the secondary filter or in the primary filter either, nor are there any wet spots under the car (at least not diesel ones ). Glow plugs work (when one goes out on this car they all stop working completely, as does the salt shaker-looking thing). Valves were adjusted a couple thousand miles ago. Tank vent is clear. Only possible thing I can figure is moisture getting in the intake from sitting in my fairly humid garage, but I took the air cleaner off and found no moisture in the part that runs from the air cleaner to the manifold, or in the manifold itself.
At my wit's end here, any suggestions?

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1973 220D ~135k Mi
1979 Ford F100 300 I6 - 225k Mi