Sounds like a bad contact in the lens, as such only a faster shutter speed would trip the flash sync hard enough to make contact. I'm in Grand Rapids area, you could always have me take a look at it (either on a personal basis, or can bring it into the store where I work cameracenter.net in the same area).
But ya I'm thinking thats what it is, because normally it should fire all the time at any speed.
That what they calling it down there? But ya its one and the same, off the corner of Wealthy and Lake Dr, with the thousand or so old cameras along the walls.