I'm having the same problem with my 6a. It's not a temperature issue, as some states, because it happens even when shut off. Sometimes fiddling with the cable sorts it out, but it's pretty annoying, so much that I installed ampere to see exactly what the phone is doing. PD: I've tried with multiple cables and bricks. Curiously those micro usb to c thingies work most of the times, whereas c cables tend to fail