Ok, I cannot figure this one out for the life of me. Two weeks back I had been driving the gst all day before work...when I pulled into my parking lot, I noticed the temp gauge creeping towards the red. I immediately shut it down and let it cool off. I noticed that I heard coolant bubbling in the overflow tank. So I checked that and sure enough the overflow tank was full and a ton of hot air was being pumped into it(causing the bubbling). So I thought it might have been the thermostat getting stuck shut...had it changed the next day. Things seemed to be going well after changing the thermostat. I noticed a couple times that I still heard bubbling in the overflow tank...but didn't think much of it.
Fast forward to today, I drove the eclipse maybe a total of 50 miles...pulled into my driveway and parked it...noticed that while the turbo timer was counting down my temp was going up...so I shut it down immediately. Once again, the coolant overflow tank was overflowing some and a ton of hot air was being pumped through the return line into the overflow tank.

Main facts:

No other symptoms of leaky or blown headgasket

No oil in the coolant

No coolant in the oil

Both times the car has had this problem, after it's had a chance to cool off, I check to see if the coolant has returned to the system out of the resevoir...both times it has.

Temp remains a touch under half way under normal driving

Only seems to overheat once I park it after running it semi hard for 50 or so miles

No coolant leaks anywhere that I could see

Thermostat was changed maybe 500 miles ago

Water pump was replaced in January

Nothing blowing out of the exhaust upon start up

Both fans are working properly

Cooling system was flushed a few months back

No smoke whatsoever

Oil pressure is perfect

Temp sensor has not been changed since I've had the car(which leads me to believe it's never been changed)

Radiator cap looks fairly clean and new...but also has not been changed while I've owned the car

Mods to be noted: dejon piping from smic to intake mani, dejon intake pipe, type rs bov(recirc'd obviously), hallman mbc, 5zigen catback, autometer boost gauge.
Running ~13psi right now


Only things I can think of is that I've either got an air pocket or something else trapped in the system or the radiator cap has taken a shit on me
Throw some ideas at me...I'd like some input before I do anything else.
Thanks in advance for any input!

2007 wrx