You will find the voltage to the headlight has just dropped a little and it is no longer enough to power the globe. A few revs and the dynamo kicks in and no more problems. It seems to be a problem with some of the 6v led globes as they only work from 6v or a bit more up. The globe on my VB used to flicker at idle. My battery condition has deteriorated a little and now the light cuts out completely at idle but comes back with a few revs.
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