I was having similar issues until I stopped holding the glow plugs on for 10 seconds. Now I hold them on for 6-8 seconds and allow 3 seconds delay before attempting to crank. I used to always try starting as soon as I released the glow plugs but there was always one or two cylinders that seemed to sputter, so I fell into the trap of holding the plugs on a little longer, thinking they were not getting hot enough.
My 1986 style controller rarely works properly because it often goes straight into after glow cycling. What was very strange is that if I allowed it to do this for 3-4 short cycles, the engine always fired up perfectly on cold start (even better than 10 solid glow). That's when I realized that waiting a few seconds after the main glow event might help by giving the prechambers time to heat soak. It seemed to work, and I've had no issues since.
I do have a solid state controller that I bought off Ebay ages ago. Just no time to put it in, and I'm still not 100% sure if it can be made to work with the spade style ZD1a style plugs. There seem to still be conflicting opinions on that.