Sounds like it is either in the wiring or the switch to the front brake lever. I would make sure the wires were making good contact and then if the problem persists I would take the switch apart to try to fix or replace the whole switch.
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Double Darkside - Bridgestone Battlax BT45 130/70/18 Front, Achiles Platinum 195/60/R16 Rear (GL1800)