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1-5 is a great start, but sometimes the problem is hardware related.
6. Check connectors, Yes you need to open the case for this. Sometimes, although this is rare any more, connector get corroded and loose contact. Pull cable out and reinsert them 2-3 times to clean them off. Use contact cleaner spray(not WD-40)

7. Reseat any cards in slots. The same principle applies with card as does cables. Memory modules may not give errors, but just function very slowly.

8. Clean or replace the fan. An overheating system may not crash just go slow. This is true especially in laptops.