After three years with a Silvia, and doing everything "right," I still wasn't getting perfectly consistent shots. I use a Rocky grinder, my Silvia is PID'ed, I use a naked portafilter to verify things, and I backflush on a regular basis. Two weeks ago, it occurred to me to sift the coffee grounds as they come out of the grinder. This has produced amazing results. I use a small sifter (not too fine, or it takes too long to sift). Exact tamp pressure doesn't seem to be that important, although ensuring that the PF is filled and leveled at the brim does. Sifting the grinds ensures that the coffee distribution in the PF will be absolutely uniform. After two shots a day for two weeks, I have yet to pour an inferior shot! Each one is a little miracle. I had resigned myself to unpredictability, but I finally have it down. You can actually get perfect results EVERY time!