Maximus, Draco, did you guys find a source of your problem?
I just got a 2006 STI, second time I took it out for a drive, I noticed the temperature was fluctuating a bit, so I checked the coolant reservoir, which was full, then the radiator which was missing some coolant.
Alright I thought, No problem! Just fill it up and Ill be good for another 50k miles.
Today, I enjoyed a cluster-**** of waiting for towtrucks, being late for work and appointments, etc., due to the same symptoms you are all describing. This is NOT what I was expecting after spending a LOT of my hard earned cash.
My mechanic initially checked over all the coolant lines, and replaced a bunch of the ones that looked cracked or old (keep in mind, this car is only 4 years old!).
No smoke out the back, the coolant and oil are clean, and there is no obvious leak.
I have maybe done 200 miles since I bought the car, and have put a few litres of coolant in! It must be pissing out somewhere?! But seemingly only under hard driving conditions?
I am giving the local mech one last chance, then it is off to the $90 per hour specialist, who is a half hour tow away!
Perhaps we could swap emails, to update each other? P.M. me if you are interested,