I think you just have a bad glow plug. My '87 had the light go out during the summer but it started without any problem until cold weather this fall. It progressively got harder to start and eventually it would take 20-30 seconds to start then stumble terribly for 10 or 15 seconds.

I ordered the glow plugs (never changed over the 100k miles I have owned the car), unplugged the wiring harness from the relay box, (the fuse was fine), removed the intake manifold (order yourself a new gasket also), and replaced the plugs making sure the wiring was intact and fixing any bare wire that could potentially cause a short.

I found that the #1 plug was damaged and the shaft of it would spin when you tried to remove the wiring nut. After replacing them it now starts easily and the light works as normal once again. I have noticed a much improved start in cold weather.

John in NC