Well since the glow plug circuit appears to be activating again once it has turned itself totally off there indeed seems to be some external to the relay problem. The relay itself normally would be suspect but your past experience and the subbing in of another relay with no improvement clears that as you know.

Yes it could be troubles in the electrical part of the ignition switch. I would read the instructions of how to remove that electrical switch component in the archives and try to get one from a junkyard. This is based on their low rate of failure. It is the tumbler section of the switch that normally fails hardly ever the electrical switch itself. So a used part is a reasonable option in my opinion.

I will sit down and have a look at the schematic in a minute and post anything I can see that might cause this other than what you suspect.

Okay had a quick look at the schematic. Will locate a magnifying glass tomorrow morning and have a better look. My eyes are older and the schematic is not printed in a bold enough fashion. The way that glow relay is controlled was not quite what I would have expected so want to examine the schematic very carefully. Do you have a schematic as well?