gears overshifting issue As I have explained in my previous post here I have changed my rear shifter cable and housing. But I'm still having issues with shifting gears smoothly. 

It overshifts at particular gear combinations or doesn't shift into the right gear. For eg., when I'm shifting to a lower gear from 5 to 4 it skips 4 and goes to 3 straight off. But this is not a problem when upshifting, meaning it shifts from 3 to 4 correctly and 4 - 5 correctly. I tried tuning/indexing gears but if I fix that particular issue above, it stops shifting into from 7 to 8 or some other gear combination stops working correctly. 

As for the bike it's a GIANT Cypress which I bought used off online. It's got shimano acera gears. I have cleaned and lubed the drivetrain but shifting doesn't Improve. I have just attached a pic of the rear cogs just for you to see the condition of it, if that helps the least.

could this be a problem due to the chain? or derailleurs? or something else? Anything I should try before replacing anything or taking it to a bike shop?
Since everything else is working, this is when I suspect hanger alignment. Where the rear derailleur bolts on to the bike, just under the quick release/axle, is a relatively fragile piece of metal. They get bent if the derailleur is bumped hard, and for that reason are often replaceable parts.

The problem is sufficiently common that there's a tool made specifically to fix it:



Those are about $AUS40 to buy, but you're not going to use it very often. Probably once. But your local bike shop will have one, and it's probably less than the cost of the tool to get them to look at the problem. Or you can buy the tool and look up how to use it (it's not hard). Without the tool you're very unlikely to improve things by trying to fake it.