Definitely sounds like the dimmer switch part of the multifunction switch. New multifunction switches can be bought from NAPA for $60. In order to test and see if everything is working correctly, you must pull your steering column apart. You will need some Torq bits, a phillips screw driver and a 5mm socket or nut driver. Take the 3 torq screws out (they are in pockets on the underside of the steering column). Then remove the 3 phillips screws. Next, tilt the column all the way down and then remove the torq screw holding on the tilt lever. You should now be able to pull the top half of the steering column cover off. You will see one screw on the very top right of the multifunction switch, a wiring harness will need to be removed from the top of the multifunction switch in order to remove this screw. The other is on the side alittle above and forward (towards the seat) of tilt lever, the lower cover will need to be held down a little to gain access to this lower screw. Then remove the upper and lower connectors and it'll pop right out.
To test the dimmer switch, you will need a continuity tester. The terminals are on the back of the multifunction switch and they are the 4 wide spades. The top terminal is the high beam output. Second down is the low beam output. Third is the visible horn input (momentary high beams). And the bottom terminal is the main power input to the switch.
If you are good with this type of stuff, I would recommend taking a read through this thread, Headlight Problem FIXED!!!
I originally fixed my headlights when they started doing this. It worked great until a little piece of plastic inside the dimmer switch decided to break and render the switch useless again... Right now my switch is set to low beams only and cannot be switched to high beams without holding the signal stalk for the visual horn. I just rigged it up so that I could still drive the car until my new multifunction switch comes into the local NAPA. The stupid thing is that the wiper module and dimmer switch module are separate little self contained boxes inside the multifunction switch. These separate modules cannot, however, be bought separately...