I don't think it's an issue with the Flash, so doing a factory reset, firmware update or clearing preset banks most likely won't change anything. I'd suspect it to be an issue with either the voltage regulation, faulty memory or the DSP itself. https://www.norduserforum.com/…ot-t20997-30.html#p143872 is an example of memory failing in a Clavia Nord Lead 3 which uses the same DSP architecture, so such a failure mode isn't entirely unthinkable.
Swapping the memory ICs out for replacement is also quite doable for someone with a hot air gun and some SMD experience - like a good phone repair shop.
http://studiorepair.com/galler…_11061601_1106213807.html shows the PCB - the voltage regulators on the left, the three RAM ICs between the voltage regulator and the DSP.
One of the voltage regulators is a switching voltage regulator which can wear out the capacitors that are part of that circuit. Replacing these would be my first course of action (like here: https://erichizdepski.wordpres…17/access-virus-b-repair/ ). My second course of action would be to replace the 7805 voltage regulator with the heat sink - who knows what wear it suffered from the heat and it's cheap anyway. If the problem persists, I'd replace the three RAM ICs. Then the DSP or the audio codecs, depending on what's cheaper to obtain.
One can of course send it also to a synth repair shop but who knows what they'll charge.