2004 Buick Lesabre coolant overflows and heater inop

Heater problem
2004 Buick Lesabre 6 cyl Two Wheel Drive Automatic

i have been tring to track this down for almost a week now and this is the problem. First off I was driving down the road when I noted that the engine was acting sluggish and the heater was not as hot. Not five min later the the engine sounded to be chocking as I applied gas and then shut off. The coolant temp gauge still showed normal thru the whole thing. Some how I lost almost all the coolant so I refiled it and limped it home where I noted smoke from the exaust. I put leak stop in it and no more smoke. Next day after I put bout 70 miles on it and noted that the heater was not working and the coolant gauge was climbing. I stoped and refiled the system and heat returned and I made it home. I hav since replaced the thermostat and fixed all leaks. Now when I fill the system up and run the vehicle it the coolant overflow tank fills beyond its max and starts to dump out the fluid and the heater does not work at all now. I have checked with radiator pressure gauges that the system has no leaks. I am concerned that the coolant gauge might malfunction again and I end up with more damage than I can afford