All page get ranked in position 6-15… all the content is optimised with internal links..
optimised how?? The word "optimised" can mean different things for different people
Anyway, I was just giving you some ideas, I obviously can't know what's wrong with your site unless I go in depth into analyzing it (which I won't do as it's very time consuming and I'd also have to charge you money, which I don't want to do)....
So, all we have for now is blind guessing
Personally, I suspect it's something on google's end, because your pages seem to fluctuate every few days as opposed to every few hours. From experience, I know that if they fluctuate hourly it's usually an on-site issue (lack of authority, poor content, cannibalization issues, etc). But since they stay up for days, then they go down for days again, and up again for days it could be a google issue, which is not surprising as I saw quite a few people saying that their sites have gone haywire too.
So, unless you manage to figure out the real issue I'd say give it a week or two more if you can afford it. Maybe the site will stabilize on its own once google fixes their stuff...
It would also help if you could check the before / after of the SERPs for your highest fluctuating pages, as sometimes google implements / tests all sort of stuff (PAA, generative AI, knowledge graphs, etc) which disturb the SERPs considerably.
Even if it's not google, checking the before / after status of the SERPs for your highest fluctuating keywords and pages might reveal interesting stuff...
If these are not the problem, I don't know what else it could be, sorry!
