I had the same problem. it was my headgasket. Pressure testing for a blown headgasket wont always work.Pressure tested mine it would hold pressure just find. But its when the motor is running you have a exhaust leak into the cooling sytem throught the headgasket. Get someone to do a leakdown test on motor. set the motor at top dead center pressurize all cyclinders. If your headgasket is blow you will see the radiator start to bubble up water or push water out of the top. I just replaced my headgasket this week. I took the head to Barnett's machine shop for full valve job and milling of head. Car runs like new now and has 220,000 on motor.