When I am faced with such enigmas I try to start from scratch. Disconnect the shift cable, take off the chain, get my book and start with step one. Often this will finally correct the problem. I find that after a few months of occasional tweaking, sometimes the various adjustments seem to be working against each other. Starting from scratch seems to get things back in line. Make sure the derailleur is clean and all pivots lubed and moving freely. Which way does it seem to shift at these times, to a smaller or larger cog? Also check the cogs you use most. IF the teeth on those cogs are worn, say half the width of teeth on less used cogs, you may need to replace some cogs. Look at a number of the teeth as some teeth are narrower than others by design.
Regards,
Raymond
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