heater blows hot then cold then hot then cold

Check the coolant level. It could also be a worm coolant pump that's not circulating water when at idle speed.

I would also replace the thermostat. If it is sticking, it can cause the problem you are describing.

If it was my thermostat wouldn't it be overheating because we drive it over an hour on a daily basis

This happened to me yesterday. I called my brother-in-law who owns a shop, he gave me a list of things to check. The only thing that I could do while driving was check the level in the coolant reservoir, it was empty. I stopped and bought antifreeze and filled the reservoir with 50:50 antifreeze:water. I started driving and immediately the problem was fixed. Also the coolant temperature had large swings from 180-232 degrees.