A couple of thoughts
I don't think you have to upgrade the wiring harness, because I think the 1990 model already has it, but if not, you should look. I think the C-30 association has links to it or catalinadirect.com. I have the same problem too sometimes, and I have the upgraded harness. It usually happens after I've been sailing for a while with the engine off and have to start it again within an couple of hours. I'll charge the glow plugs for about 10 seconds and when I push the starter-nothing. At first I thought it was a battery issue, but I'm not so sure, because I've got enough voltage. Also remember, that in warm weather 10-15 seconds should be plenty of time to hold the plugs. You will have to do it longer in cold weather. A thought I've had: the engine is still very warm when I try and start it, so I think I'm holding the glow plugs down too long. I'm going to try it for 3 seconds or so, when the engine is still warm. Also, I have better results when I open the throttle quite a bit, so when the engine does start, its at 1500 rpm, rather than 900. I never have this problem at the dock when the engine is cold, it always starts. I've asked the same question as you before, but I've never got a good answer other than 'replace the harness'.
