1999 2.4L Sunfire GT overheating

[oldsguy2]

My son's car, which I now have and he has mine, started overheating 2 weeks ago. It looses coolant from the overflow bottle. Drive it for 10 minutes, temp is just over half, sit for 5, gauge rises to three quarters from caution, fan comes on, temp goes down to just over half, 2 minutes later, the temp is at three quartes again, the fan comes on and bottle bubbles and blows out the fluid from the overflow.
Rad shop did a pressure test on the bottle which I don't believe is accurate and is now pressure testing each plug separately to see if air bubbles back through the bottle. They think it is a head gasket but can't prove it. Costly thought process if not the problem.

Could it be the water pump? The thermostat was changed.

Could it be air bubbles in the hoses,etc? They say they got them out after the first time I brought it back.

Car has 150,000 km, has run great until now and the air stopped working 2 weeks before he notice the overheating problem?

Any help would be appreciated?

Thanks

[preacher1311]

most likely it's the thermostat. but after the thermostat is replaced you need to also replace the coolant reserve tank cap