I had something similar when I joined (with my previous profile here), for about a year there were no sales. Then all of a sudden I had surprisingly good sales for a month, then it decreased again. Now it's quite stable and my monthly income is in average more or less the same. I always think it's probably just coincidence at what time people need what I sell, as my portfolio is not big yet.
Do the times with slow traffic somehow fall together with long holidays in your best selling countries?