2003 Outback overheating if I I change the temperature setting

I have a 2003 Outback that just
started overheating if I change the
temperature in the car. The first
time it happened I turned from
cool (not ac) to hot to defrost. The
temp gauge didn't move gradually,
it went from normal range to
pegged at hot in seconds and the
hot air from the vents turned cold
at the same time. Let it cool,
topped off the coolant and no
problem until tonight. I changed
the temp from hot to mid
way...almost exactly where the
red/blue meet on the knob and
the same thing happened. I had
been checking my coolant level
daily since the first incident and
it's been fine. No visible leaks, no
discoloration of my oil or debris in
my coolant overflow. I probably
put about 250 miles on it in
between events. Thanks for any
suggestions!