1999 Pontiac Montana water temp. Fluctuation

You are loosing coolant and it is causing the gauge to bounce up and down. It takes coolant to make heat low coolant no or little heat. Can you see where the coolant is leaking?If not look at the bottom of the oil cap is there a white mayonaise looking stuff on it?Can you hold your bare hand up to the end of the tailpipe [closer than 2"] without it burning it?If not then it sounds like an intake gasket is leaking. And it is burning the coolant. Most shops want $600.00 to $1200.00 to do an intake gasket job. You are just one of the thousands of owners of these motors [3.4] and most have gone through what you are two or three times. I have done mine twice. Save the paper work if it turns out to be an intake or head gasket as there is a classaction lawsuit against g.M. For these problems on the 3.4 motors 1996 thru 2004.