gears slipping

lugsey2k5

i recently bought a new chain for my kona dawg and since then i havent be able to tune the gears properly. the middle gears tends to slip and skip back and forth when under preasure which means i doesnt do it when i have it the work stand to try and fix it.
any ideas.
its shimano deore
with a SRAM PC951 9 Speed Chain

supersonic

Worn cassette and/or chainrings. many times all need to be replaced together.

lugsey2k5

yer thought it might be that the chain was very wore well over 1%. i get paid soon so new casset it is