Hi:
If your laptop is old enough to have removable ram, I’d remove one module at a time (if there are two). If you have a known good module, you can swap that in. Bad ram can create weird symptoms, and it’s one of the easiest checks you can perform. I would do this first.
If your ram’s good, it’s just possible your hdd is going bad. This is harder to diagnose and fix. My younger sister’s 2019 Macbook just had its ssd fail, so it does happen on occasion. Unfortunately, newer Macbooks have everything hardwired to the logic board, so the entire board has to be replaced. If your laptop is older, you may be able boot from your dvd drive and run some tests that are more thorough than Apple diagnostics. And it is possible to replace the hard drive with care and diligence.
One other possibility, again, fairly hard to do, is to re-apply thermal paste to your CPU. The paste can get hard with heat and age. Usually as it fails though, it will throttle your speeds down to prevent damage. So, less likely than the others to create the symptoms you’re experiencing.
Good luck!