Have you checked your oil? you want to send in a sample for testing to see if there are traces of anti-freeze. Any heavy truck repair place should have this kit.
If the system is hot, carefully release the pressure in the rad till its all gone. Start the car and run it. It should not build up any additional pressure after its hot. Thats 1 way to tell. You can hook up a pressure tester after its hot and see the pressure climb. If it does, then that could be compressed gas getting into the coolant.
Look for perculating into the expantion tank also. That could be gas getting in there also.