I have a 2002 Subaru Impreza Outback Sport. What could be causing it to shake when I'm

[Joe]

driving on the hwy? I haven't had any big problems before and this happened for the first time during my commute (about 35 miles each way, mostly highway) a couple days ago. On my way to work in the morning, my car, including my steering wheel, shook pretty good when I was driving about 60 mph - for about 5 seconds...scared the heck out me, but it went away and I made it to work without it happening again. On my commute home the same shaking occurred - but this time it happened about 5 or 6 times (about the same length of time for the shaking...5-10 seconds). Once my car is off the highway and I'm going slower, it's fine. My tires are new, so I don't think it's uneven tire wear. Anyone know what could be causing this? I've got about 100K miles on the car