Indexing problems

Bike is a Rockhopper 09, bought a new chain (SRAM PC951) after the original shimano started breaking up after around 2k miles, this slipped so bought a new cassette (SRAM PG 970) and the slipping stopped although it never seemed as smooth as the original setup.
About a week ago I started having shifting problems, gears not changing up or down smoothly and then chain coming off the low sprocket when selected.
I've checked and set the high and low limits on the rear dérailleur and the low was miles out (I suspect from new) and followed setup videos as follows:-
Set high and low travel rear derailler
put chain on largest chainring and smallest rear cog
loosen cable clamp on rear derailleur and screw adjuster all the way in
check cable is seated properly
tighten up cable clamp
index up one gear higher while spinning the crank, chain doesn't go up as slack in the cable
turn adjuster until the chain moves up the the second smallest cog

According the the instructions all should be fine but it's still reluctant to change down and often shifts two cogs instead of one (up and down)
I've taken the cable off and lubed it (it moves but I'm not sure it's ok), removed the rear derailleur, cleaned and lubed it but still no improvement. I also suspect all is not well with the shifter as it seems tight moving into the lowest cog.
I really don't want to give in and put it into the lbs but I'm running out of ideas, should I buy new cables? Also is the 951 chain ok with the 970 cassette?

Thanks for taking the time to read the post!

I've tried to follow the park tools guides listed by SS in his sticky but I've ended up more confused.