Is it still only channel 7 that is affected or other channels?
Have you tried swapping thermcouples between channels? Does the issue then occur on the same channel, or the new channel to which you have connected the thermocouple?
Please ensure that you are using the latest version of PicoLog (5.23.0) which can be downloaded from:http://www.picotech.com/software.html
With regards to the PC, is the socket to which it is plugged on the same circuit as the socket to which the equipment is being powered by?
Are you using the supplied blue USB cable and what sort of USB ports are you using on the PC - are they on the front or rear?