00 Pontiac Sunfire Over heating problem

Engine Cooling problem
2000 Pontiac Sunfire 4 cyl Front Wheel Drive Automatic 172,000 miles

My car has been over heating for the past month. We have replaced the water pump with no results. Replaced the thermostat but was still over heating so we replaced it with the old one nut opened it. For a while it was running fine until the humidity rose and it began to over heat again we at the time believed it was just low coolant. We tried to bleed the coolant system but found no air in it. Also there is not any water in the oil. I am looking for other thing to do too my car to correct the over heating problem.