Heat goes cold for about thirty seconds

My niece noticed her heat all the sudden getting cold while she was driving. I checked her antifreeze when engine was cool. Looks like it definitely needs to be flushed. As the car came to normal temperature once the heat started to blow cold, I held down gas a little and noticed the temperature gauge drop a tiny bit and raise back up within about thirty seconds and it started to blow hot again. The other thing I noticed is after the car was running for about thirty minutes, neither cooling fans came on and she has not had any problems overheating. I am baffled.