Shaking at 150

[yellowboxster]

Hey,

I got a 02 Boxster 95,000km and Ive been experiencing some shaking at higher speeds usually always at 150km but some times it shakes sometimes it doesnt I dont get it. I have fairly new tires and recently did a wheel alignment and wheel balance on all rims and still same problem. Also another problem is I sometimes feel the road more then other days, like when going over bumps or rough roads car feels like **************** as if the shocks were broken and other days no. Like the car has its good days and bad days and its starting to piss me off. Any ideas? Ive heard about the Lexus switch but honestly these cars cant be that bad.