I'm using a Vario grinder with ceramic burrs and a Gaggia Classic(pre-2015). I have dialed my espresso in at 2K on the Vario and i'm using 18 grams of coffee in the double basket. I understand that the Classic isn't exactly the most advanced machine on the market and some variation between shots are to be expected, but with this setup i have gotten anything from 16 grams of espresso in 30 seconds to 42 grams in 23 seconds.
I'm still rather new to this, so i suspect that it might be my tamping that isn't perfect every time, but is it possible that this big a difference is caused by more than just a somewhat inconsistent tamp?

Are you switching back and forth between brew methods using your Vario? That will cause some inconsistencies with the grind, and cause problems with your shot times. The Vario is a capable grinder, and if you don't fiddle with it too much once you find your sweet spot, should get repeatable results.

The Gaggia Classic is solid beginner machine, and the only wild variables you are going to get are temp., which would affect flavor, but likely not your shot times.

Tamping could be an issue, but most likely not.

How fresh are your beans? If they are a few weeks old, that could be the culprit in your inconsistencies.

Best of luck!

I think i've found the problem. I've been using 18g of coffee in my basket, but that amount may have been to much for the double basket. I read that you should be able to put a coin on the tamped coffee and lock in the portafilter, without the coin being pushed into the coffee. When i did this the entire coin was burried in the coffee, so it seems that the puck was in direct contact with the shower screen. I tried downdosing to 15g and already the timing is a lot better and the shots a lot tastier!