Sometimes what happens is that for a short time, your POS System stops and starts up again.
Several things can cause this, but a common cause is often a cable being loose or deteriorated.
Here it is not out as such but only slightly loose. When something knocks it, it goes out for an instant. This is because retail shops are busy places. People are in a hurry and often over time move cables often as they add more cables.
Check that your cables are firmly into your POS devices and electrical plugs if this is happening to you.
Many factors can cause a cable to deteriorate.
Age can do it, but it is more likely to be something like moisture, people moving the cable, heat or rats. I have seen them all. Over time, the insulator that protects the wire then the wire becomes damaged. Once this happens, problems show. Then this can be very dangerous.
I find a helpful check here is a visual inspection of the cable and area. Rats leave black spots. Also, check for an offensive odour.
If you think something is wrong here, you should replace the cable immediately.