Are the litte tiny set screws on the stepper motor pulleys tightened on the flat of the shaft? These come preassembled from the factory so they won’t be screws that you tightened as part of assembly. They are part of what comes already assembled. There are two in each pulley. It is not enough for them to be tight: one of them MUST be tightened on the flat of the shaft or you WILL have gremlins.
The ones on my XXL did not come tightened on the flat from the factory. This was discussed muchly on the forum at the time and C3D presumably improved their process, but it is possible that yours were not assembled correctly at the factory.
I mention this because I can’t tell you how many times folks have been sure that all the screws were tight, but they still had this specific problem.