Hi.
I feel I'm pretty consistent in my dosing and tamping techniques as I have a very similar looking puck after every shot.
The problem I'm having is that the shot starts fine, but starts spluttering after about 15 or 20 seconds.
The resulting shot has the correct taste somewhere inside, but is otherwise pretty sour.

What should I try changing?

Is the coffee fresh? What coffee are you using? What dose in grams? What volume of espresso? Do you own a bottomless portafilter? How do you dose/distribute? Are you using a portafilter with pressurized basket? Could you post a video of your preparation technique? You don't give much information to go on, but I'll guess that you're either using stale coffee or crashing the puck into the grouphead because the dose is too high (i.e., there should be 2mm ~ nickel thickness of clearance).

BTW, the recent thread Early Blonding sounds similar to your description; the suggestions there may be helpful.

Thanks for the quick response.
I'll check out the other thread, and if I can't fix my shots I'll post a video and all other details you asked for

OK, problem seems solved - both your statements were true. The coffee wasn't fresh and it was hitting the shower screen. Dosed slightly less and already saw some improvement, but now I changed to some beans I bought a couple of days ago and there is a huge improvement already.

Thanks a lot!