'99 Osyssey front end shimmy

Been fighting a front end problem for over a year. When i hit 70 mph (speed limit on FL highways) the steering wheel starts to shake. At 75 it feels like I'm driving a lawn mower. The shake is pretty constant, but occasionally seems to go away. Not sure what could be causing it, much less how to fix it.

I've been into the shop a dozen times to fix. I've replaced the tires, rotated them, got the wheels aligned, checked for bent rims etc. Car has 98K on original struts, never had any major parts replaced.

Could it be struts, or CV joint? Maybe a power steering problem?

Any ideas?:banghead: