-clkernel 3 uses 2 DAGs so 8GB are not enough to hold them and it will stop working at some moment when DAG size approaches 4GB.
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If my mem is 8 and 2x3,75=7,5 (current ETC) there is still 0,5 free mem
I guess if 4gb cards are working on -clkernel 1 then 8gb should work on -clkernel 3
Or i am missing something...