Brendan,
If I remember this correctly, there should be a wiper that runs over the wound resistance spring. The little tension spring that hold the wiper against the resistance wire can loose its oomph after years of service. If this dimmer isn't all encased, you can polish the wiper contact, make sure it is held with some tension against the wound wire, and even lightly sand the contact surface of the wound wire...or just buy a new dimmer if they are available.
Even a shot of electrical contact cleaner might do wonders for the old switch.
Stuart
