Well, I don't know why someone that Does know about this has not answered yet... luck of the draw I guess..
It does not sound like the actual glowplug is what you are having problems with... this sounds like more of a relay or timing or sensing situation... but the point is that it has probably been explanined pretty good.... you probably ought to do a search and see what you can find while you wait for someone to speak up who instantly recognizes what your problem it.. We have a really good search engine....but this will put it back to the top ...