Sounds like a classic ground problem, try tightening screws that hold
horn to fender. Fender grounds tend to be wimpy. A jumper wire from
engine block to fender might help. I'm assuming you live in humid
climate, I'm in Central Mass. and ground problems seem common, if it's
power, it works, or it doesn't, grounds have a way of corroding, than
current draw arcs them clean, so they work until more corrosion forms.