Given that photo, I don't think it's an air flow problem (Tony knows his stuff, so I'm just trying to help here), but...
I would try the cap to see if it helps.
On my IQ4 (I have 2) I don't have that large a fluctuation, you will see an overshoot when you first start, it just happens.
Do this, turn it on, leave the door closes and at 2 hours what is the temp setting saying compare to what you set it at? How about hourly after that? NEVER open the door?
But on mine, if I leave the door closed, it stabilizes with +/- a few degrees until I open the door the next time.
The other issues have to do with mechanical (fans, controllers, auger) as well as pellets (what brand can have a little influence some just burn better than others, but the IQ usually compensates automatically)