I have the weber smokey mountain smoker 18.5 in. I put a brisket on for the first time this morning, it is about an 8 pound brisket. It has only been on about an hour and a half and I'm already reading 145 degrees on my meat thermometer. The grill temperature is at 225 degrees right now but was up to about 250 for a bit before I closed some vents. I have a brand new meat thermometer so I don't think that is it. Any ideas on why it is getting hot so quickly or what to do to save it?