Are you sure this is a single pole switch -- i.e. the light switches on/off at only one location? If not, there is another 3way switch on the circuit that may be bad.
If you only have a 2 pole switch, one of the wires on the switch is a 'hot' -- i.e. have line voltage on it, and the other goes to the load (a light in this case). You need to check AC Voltage between the hot and a neutral line -- or a ground which should be at similar voltage to neutral.
Having low voltage at the switch may be due to too many other loads on the same circuit -- voltage drops as other things on the same circuit are using current. Try to turn off other loads before you test voltage.
The other thing I would try is a new bulb in the light fixture -- this may be flaky. I've had some really strange behavior out of bulbs before, and changing them did the trick.
If you're confused and still suspect a problem, call in a pro. Good luck.