I just wrote the reason two posts above...
^ mining the wrong coin, try one with a smaller DAG.
Can you please explain me a little more? i read about DAG but don´t understand the concept.
My card has 3gb Vram, is that the problem?
Your card's memory controller is actually the problem - as the DAG grows, even though it can fit it in its RAM, it gets slower and slower at random access. This does not occur with GCN 2 and newer GPUs.