I was having the same issue with CTC printer. Soon after I noticed a little shock when i touched the USB plug and the computer. I checked my voltage off of the ground in reference to V- and sure enough I had about 7 volts AC leaking from my ground. I figure data coming from the USB was referencing ground and added to much noise to the signal. My receptacle did not have a ground. I ended up adding a ground and she was happy again. Funny thing is that it operated for months with no issue. Perhaps the power supply isn't of the best quality.