This response might be too late... (ie date of your post) but check your dose accuracy (within 0.03g shot to shot) and tamping pressure (weight of about 7kg, shot to shot) consistency. Get a set of scales that go to 0.01 gram for the dose checks, and pull out a set of bathroom scales for the second.
Also cease the pour at the onset of blonding. The shot colour goes from dark brown, to caramel, to blond. Blond = bitter.
Your grind will need to edge finer as the beans age, otherwise youll find your shots blonding earlier and earlier.
Im hoping that you found the comprehensive answers you need between January and May 2009 under the section Brewing Equipment for the price category of your kit. ;)