A little more info
The switch is likely the problem. FYI, if your system is like my '77, the combination switch on the right side of the column connects the circuit to the battery positive terminal. The dimmer swich on the left controls the ground side of the circuit; which alternates between grounding the bright circuit or the dim circuit. As 83 Zwigen explained, chances are your contacts in the switch are bad. I recently installed the H-4 Xenon system in my car, and I was concerned the additional current draw would be too much for those switches (full headlight current is seen at both of those switches). To prevent an expensive switch replacement or potential fire, I wired in relays for the combination switch and both sides of the dimmer switch. Now I only have milliamps going through my steering column, and if a relay fails its only a $3 fix!
Kevin