There lies your problem then!
The temp guage has a buffer which means that at the normal operating range it will display 90. This means at any temp from about 75 to 105 it sits pointing at 90, and only moves if the temp exceeds this range.
If its not getting to 90 then either the sensors shot, or the thermostat is jammed open. Our 1.8T had a jammed thermostat when we baught it, and it would only ever get to 90 if it was left idling in a traffic jam or something, as soon as you were moving it would drop right off.
If yours isnt even doing that then i'd look at the temperature sender first.