If they seem to go off and on at a somewhat steady and predictable rate, a common cause is a pitted auto-resetting circuit breaker built into the head light switch. The fix is to replace the switch. An additional clue is the lights will usually be on full brightness of off completely.
Another common cause is overheated switch contacts either in the head light switch or the dimmer switch. Very often you will find two wires in the switch connector are hard and no longer flexible for about 4" from the connector. That is a sign they were overheated. The switch must be replaced and the overheated terminals can be cut out of the melted plastic connector body and replaced individually. Very often burned terminals will cause the lights to just dim sporadically and not go completely out. They might flicker rapidly.
A better way to approach this besides just randomly replacing parts is to connect a voltmeter that you can switch on when the problem occurs so you can determine where the voltage is being lost. I can walk you through the procedure after you remove the head light switch.
Monday, February 21st, 2011 AT 8:28 AM