The spark plugs are probably fine. 5/8 fits those spark plugs, and most cars. Only lawn mowers use the big one- 13/16 IIRC. Get a spark plug socket. If you use a regular socket on a spark plug you run the risk of busting it.
Spark plugs don't cause intermittent problems, although plugs that aren't properly gapped can cause issues- funny idle, etc.
You might just have to wait until something goes bad enough for you to be able to diagnose it. It's hard for me to say since I'm not there to listen to it.
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