Hey Old Barn Guy my GP light was acting up similar to how you describe yours a few weeks ago. I did nothing and waited a few weeks. Then the lamp would not turn on till after the engine started so then I knew the problem.

If everything is operating corectly the glow light indicates pre glow time only. A flashing glow lamp indicates a short and if the glow light comes on after the engine has started then at least one plug is not working. I then used a ohm meter to find the faulty GP and managed to change it without removing the intake manifold.The bad GP had a high resistance maybe it was in transition for a while when the lamp was acting strangely?

Small possibility it could be the temp sensor. The temperature sensor for the 60x glow plug is midway down closer to the front of the engine on the passenger side. I used to have the temp vs time graph but can't locate it basically at -20 the plugs glow for max time 30 seconds (?) and after glow is I think 90 sec. As the engine water jacket temp rises the glow time is reduced as is the after glow time.

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92 e300d2.5t
01 e320
05 cdi
85 chev c10