1999 Pontiac Montana overheating

I have heard many tales about troubles with this van, and here is mine.
My wife was telling me that the temp guage would alternately should that the engine was beginning to over heat and then would quickly drop. Along with this, she would have cold air blowing and then hot air. Naturally, I figured that the thermostat was going bad so I untook the monumental task of replacing it. Upon doing so, I tried running it and I began getting the same thing. I would get very hot air from the heater and then I would get cold air and the temp guage would climb. Then things would go the opposite way---the temp guage would go down and I would get some very hot air. Is there a sensor somewhere that I need to replace?