I'm beginning to think it's a short-circuit problem of some sort, possibly related to the metal mountings in the roof space and/or crossing conductor bars in the unit.
Looking VERY closely at the lamp unit I can see some metal conductors that pass very close to each other and aren't that firmly located. Two in particular seem to be bent noticeably more by the nominal 38mm LED festoon bulbs than the OEM filament ones as the end caps have a different shape.
This design "feature" seems to be the same on both the units I now have so I've done a bit of gentle "adjusting" with my trusty pliers to suit the LED bulbs and all clearances look good and stable.
I've also made a "cardboard cutout" to cover the top of the unit (which has exposed conductor bars) to prevent them touching metal in the roof space.
So far, so good, but I've said that before. 
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2003 D2 FL S8. Irish Green Pearl/Beige. Solar sunroof, auto-dim mirrors, electric rear seat functions and extended leather. Ski hatch retrofit. Aftermarket reversing camera. DVB-T and XCarlink now removed as redundant.
2016 Volvo V40 T5 Cross Country (4WD) with ALL the toys including adaptive cruise etc. etc. Osmium Grey with Blonde/Charcoal leather interior. Polestar performance "optimisation".  (Even rarer than a D2 S8!) Oh, and a brand new engine at just under 30,000 miles on the factory one! 
Finally: gone, but not forgotten.....
1998 D2 PF S8. Agate Grey/Platinum. Every option (I think) except electric rear seats, Tiptronic steering wheel, ski hatch, towbar & dimming door mirrors.
e.g. Cruise control, NavPlus/TV, Bose, GSM, Xenons, Solar roof, Parking sensors, Alcantara/leather everywhere of course.  (internal dimming mirror added later)
1998 (very early) Ford Focus 1.8 Zetec; ABS/TCS, Heated screen/mirrors, Aircon. Added Auto-dim mirror, Leather seats, Trip computer, Cruise control, OEM Ford SatNav with CD changer.
And before that a lot of Rover 800s, a few oddities, a lovely Triumph Dolomite 1850HL with Overdrive and way back in my schooldays an Austin Seven aka Mini 850!
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