Is your GT 520 a 2GB card?

 If so, you're hitting the "DAG file got too big to fit on the card with the other data that also has to be there" issue that is affecting pretty much ALL 2GB cards at this point.

 This is a known issue, THERE IS NO LONG-TERM WORKAROUND, there is no way to avoid it, it's inherent to the design of ETH.


 Options are:

 1) Switch to Ethereum CLassic (ETC) or one of the other "ETH clones" that have a smaller DAG file and don't have this issue yet.
 2) Switch to XMR or ZEC or other such coins that don't have this type of issue at all.
 3) Retire the card
 4) Upgrade the card to a card with 3GB or more memory on the card.