I've changed my grinder and am still dialing it in. At the moment everything is pretty much OK and inside the classic ranges - 2:1 ratio, 20-30s extraction time, 8-9 bar (not verified), visual clues, taste, but the process has two quirks that puzzle me:
1. the shot starts flowing on the 2nd second, much faster than before, even with fine grind where the machine is about to be choked
2. when I cut the pump foam forms at the bottom of the filter for a second and goes away