Sparky,
I had similar problem on my 1991 300D 2.5 and I first checked the glow plugs and did not find anything wrong. The second time I checked the glow plugs, making sure that the multimeter had a good ground is when I discovered that on of the glow plugs was going bad. I would try retesting the glow plugs, your symptons are exactly what I had.
Good luck!