I had similar issues with my Gaggia Classic and it all got down to bad grinders. I had a Baratza Preciso, purchased directly from the manufacturer (refurb) and I was seeing the exact same thing. My double shoots (18 grams) were running fast in about 13 seconds. If I went to almost 0-2 setting (finest settings), I would get longer brew's but I could tell the water getting stuck for a bit and than running somewhat burned. I reached out to Baratza on the grinder and long story short (after shipping me a 3rd grinder), I finally got a grinder that performed consitant through its settings. I am now in settings 7-8 and consitantly pulling 20-30 second shots with very rich brown crema shots. The grinder makes all the difference, as it may not be working as consitant as it should, causing inconsitant grinds. My two cents, I would troubleshoot your grinder and possibly try to narrow down the problem. (assuming you're using fresh rosted coffee beans).