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?