A mix of over and under extraction could be a couple of things, but IMO the most likely culprits are either inconsistency in the grind or pouring technique.
If you have a mix of really fine and coarser particles in your grounds, that'll definitely cause these problems. If you're grinding at home, you can try getting some beans ground for pourover from a café to see if your grinder has this issue.
As for ensuring your pour is giving you an even extraction; start from the centre, pouring in a spiral until you reach the edge, making sure to water all the grounds. You can also give your pourover a swirl during the brew, I tend to do one right after pouring the water for the bloom, and one after my last pour.
The mix in over- and underextracted might also come from me not stirring before drinking the coffee.
I honestly prefer not stirring the coffee afterwards. I like the slightly different notes on the first sip vs the last, but maybe I'm a weirdo.