Have you ever put a new cassette on it? Chain?
I assume that since it's done it since new that the chain isn't skipping, but maybe the 6&7 sprocket don't mesh well with the splines. If you've put a new cassette on it and it still does it, then it must be either inside the freewheel, or the splines are jacked.
<> Chris <>
I live in Holly & ride my
'02 FSR 26'er '09 Trek Fuel 26'er at Holdridge.