My temperature gauge was fluctuating quite a bit, it would drop down to cold and then jump to normal while I was driving, for about a week and now it does not read at all (it just stays at cold). 

 What do you think is the problem?  Thermostat? Sending unit? 

There are 2 sending/sensor units for the temp guage. Mine did the same thing and it turned out just to be the sending unit, which screws into the intake manifold. I got a replacement at Auto zone, fixzed it. Look for a loose wire clip on the end of the sending unit. If it spins around easy, it's probablu shot.