Are you sure it is jumping between gears rather than chain slipping on cassette?
If chain is correct length, cassette and chain in good order, enough spring tension in the derailleur then at least the gears close to the line of the chainring have to work! Even with a bad chainline, the higher gears i.e. smaller cassette cogs should work. If not, check for wear or just fit new chain and cassette.
If it is not slip, then start at the derailleur top jockey wheel. That guides the chain onto the cassette, so if in good order, jumping should not be possible.