HELP: 93 Toyota Corolla vibration problem!

Hi,

I have a 1993 Toyota Corolla with 265k miles on it.  There is a
vibration that has the following symptoms:
- it only occurs on the highway
- it starts gradually and increases in severity, then eventually fades
away and disappears within 10 miles.
- it happens only occasionally, seemingly randomly, and when it does
happen it happens at all speeds, not just a certain speed
- it appears to come from the front of the vehicle.
- you can feel it in the steering wheel AND the brake pedal
- it does not happen on local roads or at local roads speed
- when the vibration starts, you must bring the car to a complete stop
to get rid of it.  If you only slow down, to local road speeds
(30-40mph) and then speed up again, it will still be present.  The car
must come to a complete stop in order to proceed without a vibration
- it happens in the cold and warmish weather (here in boston is in the
40s and 50s now.  it happens in 30s, 40s or 50s)
- i put in brand new front rotors and this did not cure it.

Please help.  Email me with any advice that you can.  Thanks!

-Jon