Hmm, that could be annoying. You are using a standard prebuilt without any mods? Is it in a case?
Definately have a look at the heat thread
http://ecomodder.com/forum/showthrea...-etc-8684.html
The only time I've seen one go into thermal shutdown is when I tried a fancy vfd display.
I would leave the display backlight off during the day for starters.
An external 8 volt regulator feeding the guino power is probably the most effective mod if that doesn't help.
But if you want to tap your dimmer wire, I can put some pics together to show how to hack the guino to use that (minor bit of soldering required).