My 2001 pontiac montana is over heating one min.. and fine the next when it starts over heating the heater will blow cool air instead of hot and the resavor where the coolant goes will fill up and bubble over there is also a slite smell of rust or hot hoses comeing from air vents.I have already changed the thermostat and the shop says its fine they cant find whats wroung but something is causeing this to happen and im scared to let the wife and kids go off in it HELP.

The heater blowing cold is not a good sigh it may be that you have an air lock in the system
run the engine with the rad cap off from cold and add water and antifreeze until all the air is
gone this may take some time some times different vehicles have special bleed points in different
places but I cant help you there since we dont have many pontiacs over here
If you dont have any luck change the rad cap if that does not work take it to a mechanic
that has a snap on kit for testing for exhaust gas in the coolant if it fails this test you need a new head gasket

It could be two things...
a heater hose has worn out or got cut in someway or the heater core is bad and needs to be replaced...that could also cause you to smell some antifreeze in the car
and the over heating problem should be that there is still some air in the system after your mechanic changed the thermostat...so some air bubbles might be in the thermostat and then it gets over heated...when the air bubbles get out of the thermostat then the car stops over heating...
so let your mechanic remove all the air from the system in order for the car to run perfect...
it shouldn't be harmful to your wife and kids...but you should try to fix the problem asap :)
Good Luck!