Dash gauge won't move past 100 degrees

[cerial]

If your heater is blowing hot air after 5 minutes then your getting up to temp chack your wiring from the gauge to the sending unit and if it is good consider running a cheap gauge to check and see things.

If blowing cold air after 5 minutes throw in a new 195F thermostat and a new radiator cap.

Check your water pump for any seepage around the seals or lower hose for any pin holes. If you do the water pump flush out the engine you will be surprised the amount that will come out from over the years.