Friends of mine owned a Partenavia which, being a twin didn't get a lot of use. Pilots who used it were always moaning about randon avionics faults which could never be traced, just cost the owners a mint in trying to get 'em fixed. Then they sold it and had to deliver the aircraft somewhare in Southern Spain. As they continued their long flight south the avionics problems disappeared one by one and they delivered a fully functional aircraft. Obviously it was not used to doing long flights and this had dried out the avionics. In my book therefore, all avionics must be switched on before departure. This will minimise the problem due to damp and if some of it doesn't work then it is up to the PIC to decide whether to continue with the flight. Furthermore, any non working avionic boxes can be reported.
P.P.