We can only venture a guess without actually testing the product... but here's something to try: see if you when you recall Full Grand, immediately enter [EDIT] mode, then see whether it ever switches... If not, then the physical button [3] may be shorting - this could be simply due to dust in the machine or a broken contact or something shorting the contact making it 'think' you pressed that button. Otherwise, it seems unlikely that it would switch to another Preset and particularly to #3 each time. We can eliminate the [INC/YES] button and have to think it is something with button #3.

While in [EDIT] pressing the [3] button will not switch the program... but either way, it might be time to take it into the "Authorized Service" location nearest you for a checkup and cleaning. Do not spray any liquids or cleaners into the unit - especially while plugged in. There are electronic-safe cleaners but make sure you power down and unplug. Since it is a MO it could be 13 years old - so it may just be it needs a thorough cleaning.

Thanks for the above evaluation and the helpful tips. I came to the same result when thinkin it over. It is very likely, that the unit needs some cleaning, although most of the time it was covered by a protection foil when unused and it was rarely used in live performances. Anyway I will dig into the nearest service center and find ou what they will charge for a defect lookup and removal. I will put a note here, when I am done with the defect removal....