Same as it ever
Join Date: Sep 2008
Location: Czech Republic
County: Hlavní město Praha
Well, the temp gauge wiring is linked to the fuel gauge wiring, so there may be a wiring issue. I recall that someone once bodged an alarm fit, but wiring into the fuel wiring (to immobilise the fuel) but really don't know whether that was the issue.
Sounds like you may have two problems though, dodgy connection in the temp/fuel wiring and possibly a knackered thermostat. Luckily, the thermostat replacement is easy, and doesn't cost a great deal, so if it were me, I would replace the thermostat first and then see if the problem still exists.
You could also check the sender unit (behind the back seat) and its wiring (under the back seat), and also look at the ground points (not sure where they are).
The other (more complicated) possibility is the circuit boards, behind the instrument panel. That would be my last resort though..... and possibly one I would entrust to an Alfa specialist.
BTW, where abouts are you? There are some great specialists around the London area, so if you let us know where you are, we can suggest some.....