Highly variable capacitive sensor

I'm trying to build a capacitive lightswitch using the bareconductive paint. To start, I followed the tutorial here (http://www.bareconductive.com/bare-paint-lightswitch), although I used the Sparkfun relay and an arduino micro. I wired it up and everything seemed to work fine until I got to the point where I was supposed to program the threshold. When the arduino was hooked up to the computer this was relatively straightforward: look at the serial output, pick a number between the baseline reading and the reading when I was touching the paint, and upload the new sketch.

The problem occurs when I switch from laptop power to wall power. First, the laptop threshold and the wall threshold vary significantly. I managed to deal with this by just entering different values into the sketch, uploading it, and waiting to see if it worked or not. Certainly not ideal, but it worked fine.

My problem now is that the correct threshold seems to be changing even while the arduino is plugged into the wall. To put it another way, I will set the threshold and turn on the light. Five minutes later, the switch no longer works. I believe this is because the threshold now needs to be higher.

Right now my assumption this is an electrical (as opposed to a code) problem. Does anyone have any ideas of what I can do to stabilize the sensitivity of the capacitor?