My 820 that’s 1 year old shut off 5 hours into smoking a brisket 5 hrs into it. I let cool down and cleaned it up and then I set it to 225 and it ramped up to 550 degrees I put it away after that. Next time i used it I set it to 450 it’s takes 35 minutes to get to temp and my digital temp won’t get above 400. Why did it shut off and why does it not heat up right since this happened