Pressure switch scaled up limescale possibly, or other such debris.
Could be more terminal, internal corrosion or leaking into electrics – without inspecting you won’t know.
I had a similar issue and ended up just putting a new one in, was going to be easier than messing around trying to descale it. Bonus is the new one (same make and output) was much quieter.
From about £130 quid from screwfix…
J