Taking the RAM out, put it back in can certainly fix these kind of issues.
(But the problem may come back in a couple of months.)

Basically there are 3 separate issues here:

1. A flaky contact in the socket due to mechanical tolerances and the RAM/socket slightly shrinking/expanding due to heating/cooling down many times over a long period of usage. That may have created a bad contact and/or a very thin layer of rust building up on the contacts. Re-seating the RAM can fix this by mechanically re-aligning the contacts and/or scraping of the rust.
2. The metal of the contacts of the RAM and of the socket is usually chemically not quite the same (different alloys). This can lead to chemical reactions between the metals, that gradually create a very thin film of reaction material on the boundary between them. This layer usually has worse electrical properties than the contacts themselves, which can lead to stability issues.
   Taking the RAM out/back in scrapes that layer off and you are good to go until it forms again. Especially computers used in a relatively humid environment can be subject to this, but it usually takes several years before this becomes an issue.
3. Only applies if the RAM has been handled by people: Skin gives of a oily residue. If that gets on the contacts of the RAM it may react slightly with the metal, again forming a thin film on the contacts that effects the electrical properties.

The 3 effects above can appear in combinations and amplify each other. And they can start popping up after a long term using the computer without issues. Even in computers whose internals you never touched yourself since it came out of the factory it can happen.

Testing suspect RAM is tricky, especially if you don't have known good other system available.

Typical thing to do when you suspect a bad RAM is first to take out the RAM.
Visually inspect it for bend contacts: If there are any throw it away immediately. It will never be 100% reliable again.
Then clean the contacts and re-seat the RAM in the same slot. Then re-test.
If it still tests bad you can try a known good RAM in that slot. (Not always possible if the motherboard needs a specific combination of slots to be used.) If that also tests bad the slot itself is usually the culprit.
And you can test with only the suspect RAM in another slot.

In the motherboard/memory controller is the problem any RAM you test in that same slot will appear bad. But be ware when you change the memory layout/configuration (e.g. test with less or different size RAM strips) the problem can move to another slot. It is also possible it is guaranteed unstable in some memory combinations and stable in others (depending on the physical layout of the RAM present).

And always test with RAM timing in the Bios set to standard timing. Overclocked RAM can cause its own issues and make tests unreliable.

If you have another computer that is known to be good it is probably easiest to run that second computer with just 1 RAM from the problem system. Test all RAMs one by one. And then test the motherboard on the flaky computer by running it with RAM that has checked out the be good in the previous tests.

A few words on cleaning the contacts:
Don't try to clean the slots on the motherboard. Very easy to damage them.
The friction of a RAM strip being taking out/inserted is enough to scrape the contacts clean.
On the RAM strips themselves:
Gently rub them with a pencil eraser in the correct direction. (When you hold the RAM horizontally with the contacts pointing down you rub it from top to bottom. So along the contact in the direction of where the slot would be if it was inserted in a slot.)
Do both sides and try to avoid touching the contacts with your fingers.
如果您确实触摸了它们（或只是为了安全起见），请用棉签/棉签蘸异丙醇（可在任何药房购买），然后将其擦过触点。继续重复，直到您不再在 Q-tip 上看到任何黑色污迹。