-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.
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...
You using Win10? Which driver you are using. You can't just calculate 2x3.75.
Can you mine ETH with clkernel3? If so, then you have not enough vram for ETC and clkernel3. What driver your using.