Ben,

I had the exact same problem last year while I was living up in Oregon.
The seals on the clutch master cylinder were contracting just enough when
it turned cold to allow hydraulic fluid to bypass.  When it warmed up the
seals expanded just enough to allow them to function.

Replace the master, and most likely, the slave cylinders and you'll be
taking the long way around town just so you can hit more traffic lights!

Oh, the other thing that might work would be to move down here to
Arizona.  Always have a plan B.

Mike