Thanks to all who replied, both here and privately. I'll post something
once this is resolved, but thought I'd make an interim report.

1. The problem probably affects all loggers to some extent, not just
LX's Colibri. Someone wrote me privately about similar problems with a
Garmin which they believe were due to static electricity, as when a
lightning storm hit near the airport. The problem seemed to be worse
when the unit was not installed in the glider, so they solved it by
only keeping it out for minimal periods of time and praying that
lightning doesn't strike near the glider.

2. LX Support responded to me and wants to help figure this out. I had
already asked for help from the dealer through whom I bought the unit,
but also wanted to hear what people's experiences had been to help me
decide what path to take.

3. To respond to Keven's comment above, there is a reason to get it
resealed. It doesn't record anything when the seal is not valid!

Thanks again and I'll keep you posted.

Martin