First of all thank you for quick responses!
I will post the schematics as soon as I can, and yes CodingBadly, the relay would work if it was connected to any other digital pin as I'm using digitalWrite to turn it on.
As to the suggestions dc42 gave, moving the relay further away it's not an option as I've got to the fit the whole circuitry in a relatively small space, but the SSR is a really good hipothesis!
Also MarkT, the capacitor comes as an obvious choice for debouncing and I'm definitely trying that when I can but I'm afraid that it would also spoil the great sensivity it has right now. And the box as I said will be difficult but we'll see.
As for a software solution it isn't that simple because I can cope with the initial oscillations, however the analog readings spike continously every time the lights are on (maybe I could ignore with software some low tension values that spike?).
Thank you so much guys, I'll post the schematics later so you can have look.