My Forester (2004 2L XT) has always run in the middle of the temperature gauge - until a couple of weeks ago, since when it has started running warm at slower speeds.
Now, it gets right up above 3/4 once I've been sitting in traffic or driving at slower speeds (40mph or less) for a few minutes - but doesn't go into the red, and as soon as I get onto the open road and go faster, it very quickly drops back to it's normal 1/2.
I've checked both oil and coolant, no signs of contamination in either. I can't see any blockage or obstruction that might be limiting airflow across the radiator, and the fans are working fine.
I changed the alternator belt recently, but made sure everything was properly tightened afterwards.
What else could be the issue? Thermostat? Coolant flow through the rad?