Blue, do you have a windscreen setup around your grill?
I had the same problem with my propane grill where it would top out at about 530 degrees. Built a windscreen (they have portable ones) around it and I can now reliably hit 680.
"No matter where you go, there you are." Buckaroo Banzai
There are two types of computer users: Masochists and Linux users.
If you would like help or further details on a technical discussion we're having, email me at bnhelp (at sign) keepusiel.net .