Whenever mine get really greasy and carbon coated up they cook the best, in fact the temperature regulation is better than any indoor oven I have used it's spot on within a few degrees max one way or the other.
Trouble is at that point they are really dirty and need to be cleaned, you can balance some of the cleaning by just warming up the racks and pulling them to spray down and rinse off but it's necessary at some point to clean the whole inside of the cabinets and that's when it all gets stripped off and they run hot again.
I don't have an answer for you outside of that's just the way it works, have discussed this with CS reps in the past and they act like it's the first they ever heard of it.
I have an FEC 100 with the IQ5 controller and a PG 500, they both exhibit the same characteristics when clean or dirty with regard to temperature regulation.
My best solution has been to use Noble Chemical's Blast oven and grill cleaner, get the cooker up to temp in the 350 range and pull the cooking grates, grease deflector all other removable parts and spray them down then just concentrate spraying cleaner on the grate slides and outside edges of where they sit in the cooker and then the floor of the cabinet leaving everything else alone.
This seems to get it all back to working "normal" again after the first cook and your surfaces in contact with meat are much more sanitary leaving a good coating of seasoning on most of the cabinet surfaces.