I too have a PX7 (pretty old now huh!
) but I keep it for sentimental reasons. I once had a similar problem to this ...twice actually.
The first time the contacts on the underneath the keys needed cleaning. My dealer arranged this repair as it was quite a job.
The second time the transpose slider switch was loose and was getting stuck between positions. Tightened it up and no problem since. Maybe you could check all the slider switches to make sure the contacts are firm and not inbetween settings?
I do have the manual so if there is anything specific you want to know give me a shout.
Best to you