You can inspect your resistors up close by taking a video of them with a phone. If you detect white scaling on the shiny protective silica surface, your problem is sourced here and it is recommended that you renew the resistors.
A more difficult source to detect can be devices that generate metal dust. For example, if the metal of the burs you are using is not suitable for zirconia and creates invisible metal dust, its effects will also be reflected on the work pieces.
The easiest way to identify the source is to apply our chamber cleaning program, separate the crucible, break off a piece of the zirconia discs, clean it, and sinter it without the crucible. If the problem persists, please refer to our articles to recreate the silica layer of the resistors. It is possible to extend the life of the resistors a little by regeneration, but it is also a sign that the time for replacement has come.