my wellhead does this when it's below 20F outside. Oxidization on the wires throughout the year mean that any shift of the wire causes a disconnect.
haven't figured out a solution, i just run a drip that is loud enough i notice if it stops so i can go out and reseat the wire, otherwise the stuff outside the well can freeze and that costs money to replace.
Sounds like you just need a better mechanical connection. There are silicone-filled waterproof wire nuts available. Maybe if you clean the wires and secure them with those all will be good?
haven't figured out a solution, i just run a drip that is loud enough i notice if it stops so i can go out and reseat the wire, otherwise the stuff outside the well can freeze and that costs money to replace.