I posted an earlier question about a Grilling Temperature Question. Finally found the issue and solution. All problematic cooks were with a grill grate. In the first photo you see the configuration for the cooks that went bad. The second photo is the configuration for the cooks that went well. D 'oh! DON'T PUT THE GRILL GRATES OVER THE RTD! iT WILL MAKE YOU TEMPERARURE CONTROL NOT STABLE!