99 Oldsmobile Overheating / Heater blowing hot and cold air

Last winter I bought a 99 Alero 3.4l Automatic and have not really had any troubles with it. This summer I noticed it running a little
hotter than normal, checked the coolant and it was quite low. Topped it up and it was good for a another couple of days. But then would be low again with no noticeable leaks. Eventually the plastic 'heater hose' connector broke spewing coolant all over. So I assumed the leak was from the cracked connector. Replaced the connector and thats when I started having troubles. The temperature guage will fluctuate from Normal to half way to high then cooler than normal. The heater will blow cold air until guage reaches it's peak then blow hot air. It will continue to do this until eventually it will over heat. I'll pull over wait 5 minutes start it up and drive away with the temperature normal (not adding any coolant). It'll run good for 20 miles then repeat the cycle. I know this system shipped with DexCool coolant however I swear it was regular green coolant before I started topping it up and continued to use green.
The fans seem to be operating okay, but only when the motor is running. Never with the key off. I've tried flushing the system with cheap off the shelf Rad Flush thinking it was a clogged radiator. I've replaced the thermostat. I've made sure I've bled the system each time. Borrowed a pressure tester and the system holds at 15psi (but I dont know how how long it should hold that, I only checked it for 4 or 5 minutes) Yet no success. Sometime I can drive for 50 miles with the heater blowing warm air and normal coolant temperature... other times a short 2 mile trip the heater will blow hot then cold then hot then cold then I'll overheat. If I had hair I would be pulling it out.
Does anyone have any ideas of what my problem could be?
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intake plenium gasket is leaking and you have exceeded the operating temp of the piece of crap engine design and now the rear headgasket around number 3 cylinder is blown
Relentless99 wrote:

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If you think it is a head gasket or crack then test for exhaust in the coolant tank. Kit costs abouy $50.
If you symptoms happened only once, then it could be air in the hoses and when the thermostat opened, the air went out of the system.

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