My 1.8SW doesn't get above 70 on the reading BUT the heaters all work perfectly. I have taken it inot a garage (recommended on this website) and the guy said he let it run and it did get up to 90 and everything as fine (fans cutting in etc).
Did a 400 mile journey from Cornwall to Lake District and still the gauge didn't go above 70, outside temp was cold, around 0-3 degrees.
Confused, searched on here and saw a specialist Alfa garage which had amazing reviews by everyone who has used. I rang the guy, explained all and he said not to worry, is fairly normal in a 156.
It's one of those things where everyone says is fine yet you just wish the needle would go up to 90!!
Over 1300 miles (600 of which motorway rest pottering around Cornwall) I am averaging 30MPG which looking through threads doesn't seem too far off.
So... you can't force a garage to put a new stat in and I don't have ability to do it myself.... bit of a pickle really (and yes, i did the stat test before all this which is why I took it into garage to see about new one)
had car 2 months now and although sometimes think should have got a diesel have to say I LOVE IT!!!!