Water temp

Hey, I just posted here a few days ago. If you remember my 86 was heating up almost the 3rd mark on the water temp gauge, then cooling down, then heating up (it seems to run cooler when I'm stopping and going in town and is steady hot on the highway). I am going to try to flush the coolant this coming weekend. Tonight as it was running typically hot, almost to the 3rd hash, I decided to try something: I turned on the heat to max and the blower to full and sure enough within 15 seconds the temp dropped down to the 1st hash where it belongs. This makes me suspect that my fan is not coming on when it should, and the fact that sometimes the temp drops down from hot to normal at random times suggests that the fan kicks in sometimes but not always. Do you guys agree with my theory here? It has a newer water pump so I am hoping thats not the problem. Thanks for the help, once again.