Capacitor Touch switch with QTouch
It's possibly a dirty noisy power supply. If the same happens with a different supply, it could be that the analog circuit of your at mega is simply finicky. Atmel recommends a LC circuit to decouple the Analog side from the DC side. You need an inductor and capacitor filter, in addition to the typical decoupling cap on the Digital VCC. An extra filter cap 1000 uF on the power input might help too.