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If it works intermittently, then I would try booting into safe mode to see if the issue is app-related. If you can connect to wifi in safe mode, then my best recommendation would be to back up everything that's important to you and then factory reset. Once done, reinstall your apps one at a time. If the problem happens again, then you'll know which app is the culprit.
If safe mode doesn't solve the problem, then it's got to be failing hardware. The intermittent connections are probably heat related.