CDJ-2000NXS Master Tempo, Tempo Reset spasmodic operation

I have a CDJ-2000NSX on my bench, with the fault of Master Tempo and Tempo Reset keys randomly enabling/disabling. Closely related issues  have been going on for years with this model, as I can see from the previous posts. What I didn't see was an mention of the source. Has anyone determined the nature of this characteristic fault? Thanks for any opinions,    de AC4AQ