My guess would be the Temp Sender. Mine would work sometimes and then often never moved above the white box. What did it for me was the Temp Sender. On the 2.0 Zetec this part can be tricky to replace. It should not cost more than 12$ from Advance Auto. You will need to remove one of the hoses, this is easy. The part in question should be a 45 angle pointing down. Be sure to have the new part ready because when this sendor is removed lots and lots of fluid will start pooring out, so be ready. Getting to the part seems to be the only problem, even with the water hose out of the way. A small ratchet and deep socket should work fine with a little screaming and bad words remove and replace. Now the thing to remember is that the housing is made of plastic. So don't over tighten the new sendor.
This is a 1 or 2 hour job for less than 15$.
If that does not fix the problem then think about getting a new guage and checking the wires.
