Hi I have strange problem and I cannot find any solution. First, I am working on simple project with arduino (Uno R3-CH340, pro mini) and mp3 module (vs1053b). It is connected with SPI (a hell lot of wires, I am begginer, sorry 
). There are three buttons, two control stepper motor and one only play mp3 file. when stepper is rotating other mp3 file is also played. simple... and it is even working, but time from time, it just stops responding and i have to restart arduino.
after some tests I have found out, that I can simulate freezing simply by connecting other device like reproductor (active) or charger in nearby power socket. even when the arduino is powered by usb from pc. same thing happens when the reproductor is in other power socket, but is connected to mp3 modul with jack.
it also seems like with each "connecting" the state of arduino is worse. sometimes it just stops playing next time the playback is corrupted and after that it stops responding to buttons and showing anything in serial monitor. frustated to max, I added a blink to the sketch and it survived and is blinking all the time :). I tried mp3 example sketch, and it is freezing also.
i think, that the problem is in serial communication between arduino and mp3 module, corrupted with some kind of interferrence. Is there somebody with similar experience? Is that common behavior? What to do?
I have been moved here by global moderator Coding Badly. If coding is the problem, than I am so happy.
Three years have passed after your post, with no replies. I do hope you have already fixed the issue in the long meantime.
I am having exactly the same problem: a Nano/CH340 connected to the PC via USB, and communication stops when I plug a power supply in the mains. And the ONLY connection between the Arduino and the PSU is a single wire connecting both grounds! Inserting a RF choke in series corrects the problem. It seems the sparking when plugging or unplugging creates short transients that travel the grounds towards the PC and confuses some logic. It must be the CH340 because the atmega's Tx pin keeps pulsing.
Next step will be coiling all the wires towards the PSU in a ferrite toroid.