I have a 1977 240D, and have had MANY similar problems. The circuit (on my car) was not in parallel, which meant that one bad plug (or any single interruption) prevented the system from working. At the same time, there was a batch of Bosch Glow Plugs apparently made in India, which just didn't last. Anyway, my INDY (God bless his soul) installed a kit some years ago, creating a system in parallel; have not had a problem since, and my "fishing and hunting" car is wonderfully noisy and reliable at 386K miles. Hope this helps.