I used to get radom surge at 1/2 tank in my Wrx and almost guaranteed at 1/4 tank when out doing the rally sprints or a few laps around Collie motorplex track. Fixed this buy running surge tand and additional 044 external fuel pum. In saying this though this may not be you problem best to take car to workshop of your choice for a look over.

__________________
race tape y pipe running .05 bar till spares turn up.