2004 Nissan Sentra Temperature gauge spike and malfunctioni

In the past two weeks, my heater fails to heat unless I'm driving on the interstate at 75 mph. In addition, the cruise will work for a while, then cut off, the light will blink a couple of times and then I can resume the cruise speed.

Yesterday, coming home, the temperature gauge spiked to a very high reading at a stop sign. As I put it in gear to drive into a gas station to check the coolant, it came back down to normal.

I'm thinking it's either a thermostat problem for both or just for one. Any thoughts?