Sounds like a heater core problem. Check the carpet on the floor of the front passenger side as close to the firewall as possible. If it's wet chances are its the heater core. The guage has been moving up and down because the cooling system low on water an the guage can't get an accurate reading.
You can find info with picture for changing the heater core (if is is deemed necessary) at the following website:
Try www. Autozone. Com for the step-by step guide along with pictures for this procedure.
Once at the site identify you car under the heading "current Vehicle"
Next click "Repair Info" on the tabs below the AutoZone logo.
At the middle of the page go to "Your Ride, Your Guide" and click "Become a member" set you username and password
Log-in and click "Vehicle repair guide"
Select the category for the repair needed.(HEATING & AIR CONDITIONING)
Select (on the left) type of repair needed. (HEATER CORE)
Page opens to your repair info.
Friday, April 8th, 2011 AT 4:56 PM