From several threads here I've pieced together that
erratic coolant behavior can signal a head gasket
problem.

Here's what's happened so far with my wife's 2001
Forester, which we purchased used in 2003 and now has
120K miles on it:

Last fall I noticed erratic coolant levels in the
reservoir. (Coincidentally -- or not? -- I had had the
timing belt and water pump replaced a few weeks
earlier, as per Subaru's 100K mile recommendation.)

So she took it to the dealer to have that
free recall work done, which I understand involves
an additive put in the radiator. Then throughout the winter
I monitored the level carefully -- of the reservoir,
that is, which remained oddly over the "full" mark.
But then on the first really hot day this year, she
reported that the temp gauge went almost into the red. I
discovered that the radiator needed about a gallon
of coolant, even though the reservoir level was
still over full. The hose from the top of the
radiator to the reservoir seems to be okay.
What's going on? What else should I look for, and
under what engine temperature conditions?

--
Charles Packer
http://cpacker.org/whatnews
mailboxATcpacker.org