My heater blows heat sometimes and then it get into cold air. The needle on my thermostat goes up and down from the middle to cold or from cold to the middle. What do I have to repair? Is it the thermostat or the heater blower motor?
A blower motor would not be causing large swings in your temperature gauge readings. Looks like your thermostat to me.
When you move your fan switch to " on" you will hear the blower motor and feel air coming out of the vents - cold or hot - feeling the air is a sign the motor is working fine.
Your heat is intermittant because when your driving, and assuming your thermostat is broke, water temperature cannot properly build up and so you get cold air - most likely- you will get the warmer air when your sitting idle for a while.
In some cases, sitting idle for long periods of time can cause overheating with a broken thermostat as the water in the radiator is not given a chance to cool - I have seen this on an older ( late 60's ) car that I had.
Just make sure you keep your eye on the temperature gauge. An engine running cooler than normal is far much better than hotter.
Yes, try the thermostat first - and also check your coolant level ! ! !