Hey Dave Just in case;
When it happens unplug the adapter and go on the battery, when it goes away with the adapter unplugged tell me and let me know how you solved the problem!
I have exactly the same problem with my compaq laptop. so I just unplug the adapter.

I did not find a solution yet.
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Keyboards/Sound Units: Kurzweil 2600S, Roland VR-760, Acces Virus C, Roland G-800, Akai AX60, Minimoog, Machine Drum, Roland R8-M, mediastation x-76