YS640 temperature fails

Good morning everyone
My YS640 is giving me some grief. I set my temperature at 380 yesterday to cook some burgers. At 310 it was showing that the temperature was reached. I went with it because I was in a hurry. It eventually climbed to about 320, then started falling a couple degrees every minute. I finally shut it down and went to my gas grill to finish the cook.
I'm burning lumberjack pellets, and I clean the unit out before every cook. I replaced a thermocouple about 2 dozen cooks ago. Build date is 4-14-14.
Can anyone help me diagnose the issue?
Thanks
My YS640 is giving me some grief. I set my temperature at 380 yesterday to cook some burgers. At 310 it was showing that the temperature was reached. I went with it because I was in a hurry. It eventually climbed to about 320, then started falling a couple degrees every minute. I finally shut it down and went to my gas grill to finish the cook.
I'm burning lumberjack pellets, and I clean the unit out before every cook. I replaced a thermocouple about 2 dozen cooks ago. Build date is 4-14-14.
Can anyone help me diagnose the issue?
Thanks