Having same issue and it is the cable going from computer to power box. Noticed it by the switch stopped working however if i hold it just right it works, i need a new cable, but since u said you checked that (if I understood you right) I would ask if you have a bit runner and if so check your connection into the board mine came loose and caused disconnects so i had to re-solder the connection, I tested it by simply turning the bitrunner as switch to on position and lightly move the wires it would cut in and out and then disconnect, more vibrations when cutting wood like your saying would cause this as i didnt have problems when air cutting either.