Are you sure the plunger in the master cyl is returning all the way?
Yep, lines and calipers were my thought a few years ago. They have been replaced with new. I am going to rebuild the calipers this time. I am going to go ahead and replace the MC also. My thought on it, is it may not be releasing pressure. It's just odd it only does it when it starts getting hot outside and then not all the time.