Chain Slips in certain gears
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.