I responded to Tom via message, but thought I would post here as well. Sorry I'm late to the conversation. I don't come here as often as I should.
In the IM, I mentioned WiFi, battery, and sun on the LCD are generally the most common issues. From the conversation above, I am thinking it is the LCD. The LCD is a very thin, but solid black membrane, with the GoPro motherboard directly behind it. Sun will generate heat on this surface rather quickly. On hot days where the sun is really hitting the camera hard, we just place a ball cap over the camera. While I cannot scientifically confirm that a St Louis Cardinals hat works the best, it is the one I have tested the most successfully.
I have tried fans, but have not found them to cool enough to offset the noise they produce on the final video. Additionally, if you use the external battery to power the fan, it can generate some strange RF noise on both the video and audio. So if you go that route, make sure to use an isolated power source.
Josh