Nano USB power sensitivity?

I've been using an Uno for a while but just switched to Nano for the smaller form-factor, I bought three Nano clones and everything has been going well so far.

Then I noticed some strange things happening - two different Nanos powered by USB from the same PC, plugging in or unplugging one causing the other one to reset. They are not in any way connected except by sharing a power source.

Also using SoftwareSerial, if I power one device from a pc USB port, but then switch it to a mains-usb adaptor, I start getting far more corrupt data in the stream.

Is this kind of sensitivity normal or have I bought some bad clones? I never noticed it with my old Uno.

Regards,
Rob

Just to follow up on this, I happened to buy some different Nanos and the usb port is noticeably different, you have to push the lead all the way in before it supplies power, whereas the old ones would power up just as the usb cable was touching the port.

robfocus:
Is this kind of sensitivity normal or have I bought some bad clones?

No, it isn't normal. It is, however, a lesson in: you get what you pay for.