I've got a 2007 Outback 3.0 H6 with 170,000.  I live in the Phoenix Metro Area, where daytime highs have been exceeding 105 degrees for the past month.  When outside temps are below 100, the temp gauge is right in the middle, where it should be.  But when temps are above 100 and I'm driving on the highway (65+ MPH), my temp gauge creeps up -- not to redline (yet) -- but around 3/4 to 7/8 of the way there.  

 

I tried draining and refilling the coolant, but no change.

 

1) Should I be concerned or is this normal?

2) Should I try a cooling system flush (like Prestone?)

3) Could it be a thermostat issue?

 

 

Thanks

Things to check:

Condition of the radiator fins.  Are they clean? are they all still attached to the tubes?

Do the fan/s run?

 

I do not have experience with this model. 

 

A thermostat could cause over normal temp, if it doesn't open as far as it should.

Partially clogged [on the inside] radiator tubes could also.  Was there any evidence of gunk or build up in the coolant you changed?  In the overflow bottle?

It wouldn't hurt to check for bubbles in the recovery bottle, just to rule out headgasket problems.  This is done once the engine is up to normal operating temperature.  There should NOT be a steady generation of bubbles in the overflow bottle.  It could be a slow as 1 per minute, or continuous.

I would be extremely careful about checking the coolant level until you find the cause of this problem.