When you test the glow plugs you aren't looking for continuity, you are looking for low resistance, .6 to 1.1 ohms or so. Also, some folks have measured the resistance of their plugs, and they checked out OK, but still didn't "glow" very strong when they removed them for testing.
Since it works fine when plugged in, I think that rules out a fuel leak. Properly adjusted valves will help of course, but I'm not sure if it could be that based on starting fine when plugged in. We are assuming you have adjustable valves, but you didn't give the year of your 300D. We are assuming M617.
Good time to update the sig.
