I would try looking at the fuel pump, but change the fuel filter first. Check its got the right plugs too, ST170's have different plugs to the 2.0L zetec and 9 times out of 10 you get Zetec ones fitted. Coil pack also has a habit of playing up. At them high revs the VVT and Dual stage manifold runners switch over, could be the VVT controller, but if there is no fault code its very difficult.