97 Heater cuts out? Help

Hello i have a 1997 GMC Sierra it has about 142,000 miles and still runs great. I have been having one issue that is rather bothersome however. My heater cuts in and out as i am driving. If i leave it on when i turn the truck off and then restart it sometimes the heater will come on and sometimes it will turn off. Also, It now when it does work, it works only on one setting (the dash on the dial right below 'HI') I live in Minnesota and heat is a necessity. Any advice or comments would be very much appreciated thank you and have a great day.

I had a similar problem to this, what it came down to was the radiator was low because of a water pump leak, and this gave erroneous readings to the temperature sensor, which in turn made for very weird unexplained happenings with the heater, really bizarre. Check the overflow tank and radiator levels. hope this helps.

Being a fellow owner of a 1997 GMC C-1500 Sierra, I can safely say with no word of doubt that you should buy yourself a new blower control switch available at your local aftermarket parts store. My blower did the exact same thing. When I finally replaced the switch, I noticed that some of the contacts were fused and burnt. After changing the switch, the fan works just like from the factory.