This sounds like a loose neutral or switch wire, as the same problem exists in more than one part of the circuit. Would it be a coincidence that both the dimmer and kitchen switches are faulty? As for the buzzing dimmer, it could be a) a cheaply made dimmer, or b) the dimmer is underrated for the load that it switches, i.e. rated at 250 watts but switching lights in excess of this.
hope this helps
baz
baz, November 2007