It's not unusual to boost well over 3,000 - even up to 3,300 but I haven't heard of 3,400 before. I am guessing your temperatures are very good, considering the card has boosted that high. Have you done an undervolt by any chance?
It does however seem like you have found a good solution, at least for now. You could try increasing your offset to -200 or -100, since it is normal to boost high when power and temperatures allow. See if it's stable then - don't want to leave any performance on the table.
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