Ok... thanks for these responses. That gives me something a little more solid to go on here. With the grill temp being in the 375-400 range, I could be transmitting heat into the probe that doesn't exist inside the meat. Maybe I need to wrap the exposed portion of my probe in a little foil or something to see if the variation goes away. That would also help to explain why I didn't see the varation when cooking in the 250 degree range. There was not enough external heat to transmit down to the probe inside the meat....
You guys are too smart :) Why dinnI thinkadat?