Exact same issue apart from not being able to get to 600f i also had lots of sawdust i couldnt smoke anything less than 300f without it dropping more 100f, i bought an ash hoover to clean it all out and did a factory reset upto now its been fine and seems to be back to normal, it looks to me when its in shutdown its churning sawdust back into the auger and thats whats stopping the pellets flowing as its clogged as the dust never gets pushed out