Arduino Grounding Issue
Seems like there is a problem in your wall wart, actually some people are having problems using wall wart with arduino. The problem is with the circuit of thre arduino regulator and with sime ripple variances with de wall wart, i sugest you to setup in your project a regulator (7805) and see if the problem is gone. This is by far problem not just with grounding, but with the power suply in general. I was used to make PIC projects and once it was happening exatly the same that happaned to you but the microcontroller started to work when i put my hand or finger close without even touching it :x. Crazy thing. If the power supply isn't well suited, the arduino enter in continous reset looping because of supply noise. So just put an external regulator, an maybe an extra capacitor (generally 10 nF) in arduino micro power supply rais (VCC-GND). Here's my tip. Good look man, tell me if it worked :)