Recently bought a new grinder and still having inconsistent shots, not as inconsistent as the last grinder--but inconsistent. I'm beginning to think it is pointing to my Gaggia Classic. It doesn't have a PID so I really never have any idea where the temperature is at. I typically wait at least 5 minutes between the first double and the second double. Without knowing where the espresso machine's temperature is at, if it is too low or too high, would that cause espresso extraction to be too fast or too slow? Thanks.

pls define inconsistent, is that between two consecutive shots or over several days, even weeks? Do you weigh your doses? Is your coffee reasonably freshly roasted?

With the GC you'll need to do temperature surfing for best results, waiting 5 min is nice for recovery but you probably guesstimate at what point of the temperature cycle you are, if you hit the button when the coils just switched off temp will be max, if you hit the button when the coils just started another heating cycle temp will be lowest in the band the thermostat controls, hysteresis is quite large.
When I had one I would wait until the heater light clicked off and then hit start, at least that is what I recall...it's been a few years.

It was better today, got two very good shots. Just got this new grinder (Rocket Faustino) on March 1st. It was inconsistent between the two shots, from gusher to up to 17 seconds to first drip from March 1st to March 3. I've weighed the doses of grams going in, but not out. I turn the grinder on between shots to get retentive grinds out. These particular beans were 13 days post roast when I started using the new grinder--as my old grinder broke down & needed to order and wait for it. The beans were 8 days old when I was going to start using them with the old grinder.

Thanks Jeff, I thought I heard that/read that somewhere.