Thanks a lot, that was very helpful! Any temperatures you usually shoot for on your second-generation machine?

I like the temperature at the marked location on the second generation machine to be about 90C.

With my 2003 Millenium Europiccola, I mounted a meat thermometer to the group a bit higher than were rpavlis measures his temperature. I target around 92-94C and pull.

What I found is that from a cold start, I wait for the boiler to reach pressure and the Pstat shuts down and then pull two shots with only the spouted PF installed. The first, I allow roughly 3oz of hot water to flow out into my cup. The group warms and then settles to about 80C. I dump the cup and pull the second with only 2oz hot water. This nicely heats the cup for the final shot and the group usually overshoots my target 92C, but falls within a minute to about 92C. I then add my basket of tamped coffee and pull my final shot into the warmed cup. I am not sure this procedure will work the same for your LP, but until you have some sort of thermometer to measure the group, you might give it a try. If the shot still comes out sour, try a quarter or half pull to further warm the group head.

I like rpavlis procedure better than mine. It saves boiler water use, which would help with multi-pull operations.
However, mine warms the cup better.