This problem is caused by external power problems I would suggest. Is the cash register plugged into the socket with another appliance with a motor? I.e fridge, freezer, vacuum cleaner. If so the cash register is getting power spikes. Either put the cash register in a different socket or try fitting a spike protector.
Alternatively, Go to this link http://www.offer.uk.com/Links.php and select Looking for answers to your cash register questions. Or goto http://www.ycr.co.uk/faq
John Hall, March 2006