top bar? i had something similar. watched youtube hd video, stopped and left it to background and started bounce and switched back to browser. problem vanished by rebooting.

e: i suggest to create linux livecd(cd or usb stick doesn't matter) + create usb stick that has fw binaries + flasher deb just in case the NSU doesn't work. in that case those preparations will make your life very easy compared to other options.