Z grill temperature control

[Ja20]

I bought the z grill 450-A I used it the first time yesterday had a lot of trouble with the temp going wayyyy up. I would set it at 250 it would go up to 300 or higher for 20 minutes or so and stay there until I opened the lid to bring the temp down. Even after I opened the lid sometimes it would hold right around 300 with the two set at 250! Is my temp control not working right? Is there something that can effect the heat? Any help would be appreciated!

 

[Winterrider]

Check temps with digital thermometer, maverick, thermpro , Inkbird or other reliable to compare