The problem stems from the ST using a returnless fuel system. Have you gone over to a return system now or blocked off the line?

Either you need to go for a return system, in which case you will need to run a new line in and replace the pump with a normal 2.0L one and remove the pump controller (see ETIS for location - from memory it's against the inner sill on the drivers side, under the carpet).

OR you need to fit the pressure sensor so that the pump knows what to do

HTH
Chris