Pumping over needs to be sorted as this is what introduces oxygen into the system and leads to corrosion and blockages. Are you sure you cleared all the gunk from the pipes, particularly the area between the vent and cold feed connections? If its pumping over, the water is finding it easier to go up the vent pipe into the F&E and back down the feed rather than straight along the pipe. This suggests its normal route is still restricted. You could also try combining the feed and vent pipes as Johntheo5 suggested. This involves only having the 22mm vent connection from the F&E into the system with the feed Tee'd in to the vent higher up.