The reason that a hand held GPS "just switches off" is likely to be tarnishing of the battery contacts. This results in momentary disconnection even when the contacts are tightly pressed to the battery cells. Strangely, GPS software does not seem to be designed to withstand this highly predictable event.
This contamination is usually the result of leakage of vapour or even liquid from defective batteries and is an argument in favour of buying premium brands, which even so can still leak. (Rechargeable batteries are a false economy because they have vents to relieve pressure).
The only real fix is contact replacement, but a short term substitute is a fibre glass pencil available from hobby shops and electronics suppliers used to burnish the metal (and the batteries).
In flight, removing the batteries and burnishing them against some suitable cloth like your trousers is likely to help.
If you get this problem once, you will get it again, possibly at a really inconvenient moment!