Is the CafeSeries grinder the EM0480?
Where do you get your beans from, and how long ago were they roasted? Unless theyre from a roaster you can see (ie. not mass-roasted) they could vary from batch to batch.
Also, each batch will require a slightly different grind point, and that changes every few days as the beans age. Your single-wall basket will be less forgiving of any grind, dose and tamp issues. Do you have access to a naked pf? This will show up extraction issues.
How much coffee do you extract in how many seconds?
If the burrs are worn on the grinder it could also be producing different particle sizes, burning some and under-extracting others.
Hope this gives you a starting point - Im sure the solution is somewhere in the answer to those questions!