Not being funny, but that sounds like an old MK2 Escort pinto bodge job fix These newer cars, especially the Focus RS shouldn't be that tempremental, especially where boost is concerned. Its gotta be something along the lines of a sensor, injectors, plugs, regulator etc. More of a component as apposed to low fuel because unlike an alloy fuel race tank with just a foam insert and a 044 pump, the RS tank isn't a homemade type of thing