skipping on new chain/ cassette
I have tried twisting the link concerned (to iron out the stiff link -this will work for a day at most) and I have taken out the link (PLus 2 on either side) and replaced it with unused links from the original packaged chain (again this will work for a day or two and then back to the original problem)


It sounds as though whatever you're using to join the chain is not working properly. Are you using the Shimano snap-off pins when joining the chain, or are you pushing rivets out then back in again? If the latter, your rivet tool is apparently not up to the job.

This question discusses using the tool properly and has other tips.

My suggestion is to buy some SRAM power links or other tool-free chain joining links and use those instead. They're useful of themselves, but the real advantage in your case is that they don't need a tool and I've never seen one become a stiff link. They're only a few dollars each, and while you need two that's still cheaper than a new chain and cassette.