Intermittant vibration at high speeds

["Craig D. Niederst" <niederst@telerama.com>]

I've been on the road quite a bit recently, and a nasty vibration shows up
intermittently at around 80 mph. It does not occur all the time, and I
cannot replicate the conditions that make it show up (turning in a bend,
rough highway, etc). The shaking is very violent, almost bad enough to shake
the wheel out of your hand. The vibration, from what I can tell,  appears to
be coming from the right front wheel. Tapping the brakes stops the
vibration, and if I immediately reaccelerate back up to about 80 mph, it
does not reoccur. I pulled the wheel, and I cannot notice anything loose or
broken. The wheel is in balance (this has now showed up with both my snows
and now my summer tires mounted). Any ideas or BTDT? TIA.

Craig
'92 100S (80k)