WordPress page gets slow till error 5xx

Hello,
I have this problem since july and i am trying everything to solve it without success ??
Last summer my website started to get slow day after day till error 5xx appeared with the website unaccessable.
After several open tickets with my Hosting provider, at the first time they restored it from the server site. But everytime the situations was getting worst with more and more shorter times till my website got unaccessable.
After more tickets they have stated that it was because of some plugins (checked on the domain log) but after deleting the plugin, the log displayed another plugin as error and so on.
After the provider called me to tell me that something is saturating the server/cluster resources and its probably my wp theme fault because it was customized. But the website was running perfectly and smooth on the “staging” ambient. So my thought was, it shouldnt work also on staging so i dont get it…anyways i decided to rebuilt my whole website with a new and optimized theme…after 1 month the new website was finally ready on the staging (working perfectly) and i decided it was time to sincro it with the production domain and officially reopen it with a more cleaner graphic and more functions. But after some hours ot was online, the same problem againg: website getting slow till i hit the server 5xx error again. More tickets opended till one day ive got an email from my host in which they say that my domain was suspended because of a malware. Thats strange because i did my whole site new from skretch. They called me and we decided to make a backup of the site (run a antivirus) and reset the domain deleting everything keeping the staging page. Done this we have restored the back up but once i attached the php file to the config file the page freezed and the problem showed up again. They stated that the issue is somewhere in the wp files but i dont believe this. I let the whole wp page check it by a professional and he stated that the issue is on the server side or dns. Now i cannot change the hosting because i have several other websites on their server and they are working perfectly. Any clues?? I really dont know what else i can do….really need help from you guys.

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