Ive had this issue on almost all series 60 phones I have had. 6600,6620, 6682, 6630 etc. In all cases the ringtone was on the memory card (not nokia but 3rd party manufact.)
Upon moving the ringtone to the phone memory the issue was resolved. Something about the read speed of the memory card. This would come and go and having the ringtone on the phone memory is not my fav thing to do on phones like 6682, 6630 since they have low built in memory