Seems a so called "cpu clock amplitude"/"clock over-charge voltage" makes a very little help - limit is quite the same but 700mv make it a bit more stable just before the edge. To control this setting, you need to change byte 01 bit 7-8 with setfsb in diagnosis mode: 00=700mv, 01=800mv, 10=900mv, 11=1000mv. My board defaults it at 800mv (byte 01 reads 99). Compared to previous score, this was done with colder ambient temp (+10-12C). Valids for 685-686mhz were broken:(