Q: Heat inside car stays warm for minute then blows cold until I turn heat off for a minute and try again.

Hello, I own a 2002 Mercury sable 3.0 OHV, 125,000 miles. I just found out the heat on my car hasn't been working and it's hard to pinpoint the exact time this started because it has been summer months and just recently I've needed to use my heat. The car will heat up to normal operating temperature on the gauge and nothing out of the ordinary and I'll turn the heat on and after waiting the normal couple minutes while driving I notice the heat isn't all too warm, so I'll turn off the heat and drive a little more and then turn it on and it'll be nice and warm and proper heat level for about a minute and then go back to blowing semi-cold air (not regular ac level cold but definitely not warm air. The actual engine temperature is always at normal operating temp and the heat intensity doesn't change depending the speed I travel. My coolant levels are topped off every oil change at my local shop and this occurs every 3,000 mi and I checked to be sure and the level was normal.

My car has 125000 miles.
My car has an automatic transmission.