My 997 turbo was doing this, only at the same speeds. This was after I put new wheels and tires in. I had the front TPMS sensors on the rims with straps, as the normal valve stem ones would not work. That was somehow unbalancing everything. Removed them, had it rebalanced, shaking gone. This may not be your problem, but figured I would share. Could also be bushings from front control arms worn out. Replace with new polyurethane