Over the last day or so my car (Mk 4 1.25 Si fiesta) has started to lurch and lose power when changing up a gear (no problems when changing down). It feels like there is suddenly very little fuel getting to the engine. I am wondering if this is a fuel blockage as it was very low on fuel the other day when it had its MOT so was reved hard for that and am wondering if this has dragged sludge through the engine. If this is the case is it something that will simply go away after a while with a full tank or am I going to have to do something else. Also any ideas as to any other causes would be welcome. I hope someone here can offer some advice.