cant be many things... in my experience its allways been caused by the wheels tho (most common)... either out of balance, or one of them bent

get em balanced (take it somewhere GOOD!)

i had new tyres fitted on the front of mine recently and they musta balanced em wrong (even tho they told me they were ok) caused vibrations over the 100mph mark... got em re-balanced by SCC and now they're sweet as a nut, stable all the way thru 5th gear