Sounds exactly like what I used to experience in my MK1 Focus at 100+mph...Scary!

In my experience lowering really helps as it stops too much air getting under the car but a front splitter helped massively too, it felt much more planted at speed.

The XR4i front bumper really looks like it will direct air under the car so I reckon a Cossie bumper with splitter will help alot.