Okay here is the aftermath, whoever is interested and wants to read!!
I have been left with two options before ordering a new joystick:
1. Option: Disable joystick X function in KbdControl!
As there is no global joystick settings unfortunately, the bad thing is, you have to disable for each style separately! Means: Change style settings for a couple hundred styles and save new setting (disabled joystick X) into each style performance and most performances too! Not a joy, once the new joystick arrives, all these changes have to be reversed to enable it's proper function! We are talking of some 15-20 hours of stupid work!
2. Option: Open the keyboard and somehow disconnect the joystick itself!
Well, this would save tons of stupid work, and all the valuable time!
Here my findings going with option # 2:
Once opening the keyboard, you'll see right away what to do!
On a side note, comparing with earlier arranger models korg and roland, there is not much inside a PA800. My guess is, the next generation will weight less than 20lb, and it won't be much more inside than in a laptop
The joystick is connected with a 6-wire-cable, or better 2x3! A triple jack for the X-axle and a triple jack for the Y-axle! On the other end it's a sixtuple-jack connected to the motherboard, or whatever it's called!
Now, first I disconnected that 6-jack, turned the keyboard on, and after it worked perfectly for 2-3 minutes, right hand sound suddenly moved 1 semitone up without any logical explanation!!!! And it stayed off, so connected that 6-jack again! Resulting in again being off 2(+5) semitones!
Funny stuff, simply not explainable at all!
Ok, next thing disconnected the triple-jack from the X-axle of the joystick.
Turned keyboard on and played! Result exactly the same: 2-3 minutes everything fine, then suddenly out of nowhere again upper sound goes exactly 1 semitone off (up)! Stupid, it stays there and doesnt come back, beside if I disable the X-movement directly at KbdControl!!!
At this point I basically gave up, with the intention to connect those jacks back, close the keyboard and finally order a new joystick! Also, intention to disable X-function in styles and performances, at least the most important ones until new part arrives!
But then>>>>> MIRACLE!!!!!!!!!
As soon as I put all together, noticed immediately that the malfunction is completely gone! And the good thing after now app. 10 hours of testing and excessively playing >>> it remains gone!
Now, what did I do?????>>>>> NOTHING!!!
The only explanation: Disconnecting that triple-jack of the X-axle and reconnecting it must have done the trick!!!!!!!!!!
So, if you ever experience something similar, try disconnecting and reconnecting that triple-jack first! Must be a simple weak contact or dirt!
Maybe if I applied a hammer-fist in that corner of the keyboard the result would probably been the same
Gladly providing this info in the hope it'll help somebody, some day!!!
Still thankful for being directed in the right direction where to look!
civias