gear shift problem? transmission problem?

I have a 2000 Mazda Protege, automatic. I've been having a problem with
my gear shift; at least, I think that's what the problem is. Whenever
it's fairly cool (less than 60 degrees) and dark outside, my car won't
shift out of park. It's as if it doesn't realize that I have my foot on
the brake. Sometimes if I stomp on the brake, that will work, but not
always. Right now, the sun comes up over my apartment building and hits
my car between 8 and 8:30 am (depending on my parking spot). Literally
minutes later, my car will work fine. Any suggestions on what the
problem might be? Thanks!