So after that, you ran Apple Diagnostics and it reported "No issues found". Unfortunately, Apple Diagnostics is not the most comprehensive test. If it reports an issue then you can be sure there is one. But just because it reports no issues found doesn't mean there isn't one. Apple has much more comprehensive tests they can run if you take it.
If you like you can run Etrecheck It is a diagnostic tool that's very useful to us in finding problems. Also it will give us further specs on your Mac. After it runs post the log file here. It will contain no personal information.