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On my classic, I tend to give the temp a bit of a boost with the steam button before brewing.
After letting everything warm up, with the portafilter in place so that warms up too, get everything set up and ready to go.
Then wait for the brew light to turn off (at this point I slot my portafilter in)
Then wait for the light to turn back on (you're now at max normal brew temp, and the temp will be dropping from there until the light turns off)
As soon as the brew light turns back on, flick the steam button on for 5 seconds. This boosts the boiler temp a bit. After you've flicked it off, wait 5 seconds for that extra temp to make its way into the water, then hit the brew button.
On longer extractions (30 secs or so) this is the only way I can keep temp in the water right through the shot. Still often get a bit of sourness, but as I tend to make Flat Whites, it doesn't really matter.
You'll know you've gone too far on the steam button boost if you're getting overly bitter shots.
The thing about the process above is it is repeatable, as you always know where you are temp wise when the brew light has just turned back on.
Could just be the coffee you bought doesn’t work with your water?