I just recently purchased a 2005 STi and have noticed fluctuating water temps. Checked coolant levels and topped off, fixed minor leak, burped the system, new radiator caps and still have issues. The odd thing is that if the car is started from cold, allowed to sit for 20mins it gets up to normal temp (assuming normal temp) just above the 1/3 mark and holds steady. Driving around in town...no issues, stays in the same place. However for example doing a steady 70mph in 6th gear at roughly 2.5k rpm for a while it slowly starts to rise to just over half, if I down shift into 5th, maintain speed the rpms increase to 3k or a little over and the temp goes back down to the 1/3rd mark. Occasionally after high speed operation and coming to a stop the temp will rise, then as soon as the rpm is increased it comes back down again.
Any thoughts or ideas would be appreciated. As mentioned I've burped the system. Recently changed oil and didn't find water in it, coolant reservoir tank doen't have oil in it, great power...and the way its behaving I'm ruling out a blown head gasket. And this doesn't really seem like a normal water pump or thermostat issue either, but I've been wrong before.