Chain Slips in certain gears

From the description, odds are very high your rear derailer hanger needs to be aligned, you've got excess cable friction, or both. Both can create situations where it seems impossible to get it shifting and running smoothly in all gears.

Hanger alignment in particular has quietly become a much bigger deal over the years, as rear shifting has only gotten more sensitive to it, and many bikes have very easily tweaked replaceable hangers. If you don't want to buy a hanger alignment tool or take it to a shop, you can typically make it at least somewhat better on an easily-bent aluminum replaceable hanger by bending it around with your 5mm allen stuck in the pivot bolt.

I would guess that chain and/or cassette cogs are worn (typically middle ones go first) if you've tried adjusting and can't get it just right. I go through about a cassette a year on bikes I ride a lot, especially when I don't do a great job of cleaning and relubing the drivetrain. However, just replacing the chain can be bad if it's already jumping. It can get worse if you put on a new chain to the old cassette.

You might be to the point where you need to clean your derailleur/ jockey wheels / front chainrings of any existing grit and replace both chain and cassette.

Other less likely considerations could be: - A damaged freehub body (if you have an aluminium freehub body). - Misaligned Derailleur hanger (as Nathan Knutson Mentioned) - Poorly Adjusted or damaged shifting cables, or cables that are corroded / sticking in cable housing.