1998 Nissan Altima heater, temp gauge erratic

The temp gauge doesn't move for about 20 minutes after starting and only touches the first line or slightly above. The heater seems to work, although just on the highest blower setting. It only puts out warm air and sporadically goes cold for a few minutes.

From what I've seen so far, I can replace the resistor behind the glove compartment for the blower, right?

The radiator's full to the top, and the reservoir is about 1/4 full when cold.