First things first.
Welcome to the Zone.
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First thing you want to do is
- (1) Check for loose / corroded wires.
- (2) Check the dimmer switch.
- (3) Check the headlight switch.
(1) You may not have a problem here but but check the wires at the dimmer / High Beam switch.
(2) Your Dimmer switch could be shot. If you have an OHMs meter and know how to use it, check the switch.
(3) If everything else is ok then you probably have a bad headlight switch. My guess is this is the problem.
Let us know how you make out.