Mac, you are on the trail! First let me mention that it was an oversight that we failed to post a response to this thread: we talk with Ian a lot, mostly off line, and I overlooked this post. Mac, each key has a different capacitance influenced by its size, shape, and position on the PCB. The resistors were selected after some theoretical analysis and LOTS of empirical trial-and-error. Plus the whole system is influenced by its environment: temperature, humidity, and how dry your skin is! So we have found the following tend to stabilize the keyboard:
Experiment with a wall-wart power supply instead of battery, and earbuds or other external speakers instead of the (sketchy) on board speaker.
Lick your fingers before playing!
Adjust that E resistor down to a lower value.
Playing with the MIDI IN jack, controlling the Macchiato with an external control keyboard or sequencer, bypasses the capacitive touch keyboard all together and performs with no instability.
Regarding a metal case: Not such a good idea. The metal itself, if it contacts the PCB at any point, will affect the capacitance of the whole system. However, one user had great success adding an additional ground plane to the bottom-inside of the cabinet. I think he used a piece of heavy aluminum foil (copper would be MUCH better) and a little wire that he connected to ground at the center of the board, possibly using the center standoff screw. This gave him good stable performance. He found he had to isolate the foil from the five screws in the cabinet bottom; otherwise, when he held the synth in his hand, touching any screw would throw it off again!!
Glen