On Sunday, 14 April 2019 19:46:03 UTC+1, TheChief wrote:

Thanks Jeff

I suspect you are right about the heat issue.
Although the fitting is as-supplied, the glass shades are
horizontal 38mm I/dia
around 50mm long.
Also, being in the hall they have foot traffic regularly above.
So maybe vibration also plays a part.

Phil

The hotter the filament is, the weaker it is, so reducing voltage slightly can help resist vibration to a [to me] unknown extent.


NT