My left rear turn signal/brake light is out

Sometimes when driving my vehicle I get the message on the dash that my left rear turn signal/brake light is out. When using the blinker it blinks fast also indicting that it is out. I have been pulled over by the police for this issue as well. While other times driving the vehicle I don't get that message and it seems to be working fine. I have replaced the bulb but I don't believe that was the issue. The bulb socket looks good with no corrosion and the wiring (that I can see) doesn't appear to have any issues with it as well. I've noticed that in the morning when it is cool outside I don't have any issues but after the vehicle sits in the sun all day at work or in the driveway it then tends to not work.