Dealing with a similar problem on a mondeo at the moment. Does your Turbo have an electronic wastegate controller (Little black box with a metal arm connected to the turbo). If so, the wastegate controller may have failed. If thats the case, I'm afraid its into ford to have either a: the ecu flashed , which can sometimes clear the fault, or b; time to replace the turbo, as apparrently the wastegate controller isn't available seperately and is only available as part of the turbo assembly. Sorry to be the bearer of bad news :-(
Mark The Bus
June 2011
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix cars.