heater works sporadically on 1999 Pontiac Bonneville

Two winters ago, I noticed that my heater would quit working. Sometimes it would work when I started my car, but when I shut my car off and started it again, no heat. It didn't do it all all last year, but this winter it is starting again. Usually (but not always) when I first start my car in the morning, the heater will work, but when I stop at a store or the gas station, and start my car again, 95% of the time, my heater will not work again. Sometimes it will start back up after I have been driving a while. Sometimes when I have it on high, it sounds like the blower motor winds down, the picks back up again. My coolant level is fine and my car does not overheat when my heater is not working. No bumps seem to make it come on or go off. I am tired of freezing in my car in the winter. I took it to someone when it first started, but he couldn't find out what was wrong. I have taken it to several other places, but again no luck. Could someone please tell me what could be causing this and how to fix the problem. I really don't want to go through another winter with no heat.

Asked by for the 1999 Pontiac Bonneville