Take a look at this thread. He dose not have a the same problem as you do but he also cannot find what is wrong with his system. The thread leads to the possibility of buypassing your glow plug relay (as a temp fix) so you can get back on the road.

After-glow/GP relay need explanation, please help

I bypassed the glowplug relay on my Volvo diesel and it has worked for me. I left some instructions and a web site where some pics are posted on the above thread. I left a couple post on this thread starting with lucky #13th post.