1998 Pontiac Sunfire overheating after a ton of changes

For the last 3 weeks I've had issues with my car. First I replaced the timing chain, then the Throttle Position Sensor, Water pump, spark plugs, thermostat, Map Sensor, O2 sensors, oil change, and new air filter. I was so excited that it was finally running good, at least for a few minutes. It started overheating, we bled/burped the coolant line, refilled, and tried again. At idle, the car started to overheat, coolant was bubbling, and the fan was running constantly, with the a/c on and off, but still not cooling it down. I'm afraid I've got a bad head or something. Especially since when we put in a new thermostat we saw that for the 3 years we've owned the car, it apparently didn't have one. Are there any suggestions as to what else we could change before calling it quits?