What you describe is a known issue. The keyboard will become increasingly unstable as the voltage from the battery drops. Try a brand-new name brand alkaline battery. Further, the keyboard is sensitive to grounding. It generally works better the more stuff you plug into it — headphones, line-out, power supply. You will notice that the keyboard behaves better when you have a finger touching the far left or right edge. It will work better when your hands are sweaty, or if you lick your fingers.
Another user recently experimented with adding a ground plane by placing a piece of aluminum foil on the inside bottom of his Macchiato cabinet, and connecting it with a bit of wire to the PCB. Some time soon I intend to post his procedure to this forum.
That cabinet model to which you refer, if memory serves, leaves room for improvement. We do still hope to publish some models one of these days.