My experience is from the MX 5500, but I assume the system is similar between the two. I believe the standby position should be hard coded as the centre position. In addition, you have one viewing position that can be programmed.
Suggestion:
Turn on the TV, store a position (picture, store, store on bl 1000) different from the standby position, and turn of the set. Move the set manually away from the stored position, and turn the set back on. Does it move to the stored position now?
If not, I belive there has to be an error with the module that is supposed to hold the memory of the stand. (Which module is beyond my knowlege).
Should it turn between two positions, different from the centre position, when in standby and on, I would suggest that a previous owner have made mechanical alterations to the turning mechanism.
Sverre