Heating problem or not?

Subject:
I or my wife rather owns a 1997 Chevy Lumina with about 190,000 KMs and I'm a little bit baffled with this one. You start the car and let it run for a minute or two then drive where ever and takes forever to warm up. However, after about a few kilometres of driving the temperature gauge goes two thirds of the way to overheating. Heat is full on and you crank the fan setting it takes about a minute then it will finally send out awesome heat. But after about 30-45 seconds the temperatue gauge drops right down to cold and blows cold air. Turn the fan off and after a few minutes repeat process. Got me right going crazy and some tests have come up with nothing. Before taking it to a shop I know if I myself knew the exact problem it would be fixed right away. I lack in the whole troubleshooting aspect when it comes to cars. Any help would be GREATLY appreciated! I do not want to take the car to a shop, they charge insane prices up here north of the border!