this happens to me as well for hardcoded bios.
temporary solution is disable p0-p6 in overdriventool. or hardcode your bios to use the same clocks on p0-p6.
this must be a bug, since this is not happening on previous versions.
But I have never had this problem before and since yesterday it is permanently on this lower hashrate. Quite strange to be a minor bug in Claymore...