My 2005 Equinox is overheating. We have replaced the thermostat and the water pump and it is still overheating. It will run fine and will be blowing heat and then all of a sudden it will blow cold air and start to get hot. What else could be the problem? When the fluids are checked, there is plenty of antifreeze in the reservoir.

[check fans possible bad sensor or fan motor ]<<< most likely problem/or radiator and reservoir may need to be flushed the 05 was known for having radiator hose problems the hoses in some would deteriorate causing a smell and overheating problems --the other 05 problem was head gasket which can cause the overheat problem but u would have other symptoms rough idle ,steam from tailpipe,oil level high and looking like chocolate milk the 05 problems usually caused the problems early in ownership within 10,000 miles and recheck stat and pump new doesnt mean free from defects

Few things can go bad.
Try another thermostat
Make sure there is fluid in radiator not just reservoir
Make sure that cooling fan is working
Check for oil in coolant and coolant in oil (blown head gasket, cracked cylinder ...)
Flush cooling system to remove all deposits from the system

Headaskets are known to be bad on these 05-06 chevy shouldve recalled em
Dont buy any chevy vehicles

Hi, im sorry to say but the problem could be more serious it could be that your head gasket has failed and needs replacing, the only thing you could try is drain all the coolant then make sure that if you have an air bleed screw on one of the pipes that this is open then just trickle the water into the coolant system doing this ensures you have no air locks.
Once the coolant system is full then give it a try and see if it over heats, you may find that its just an air lock thats causing the problem but if it still over heats then your going to need to get a compresion test done or a sniff test to check the coolant for signs of engine gasses from the cylinder.

Let me know how you get on or if you need further assistance ok