You may simply just have to shampoo daily. Many of us have found that going without daily shampooing leaves to a itchy oily and occassionally cradle cap like syndrome. Stopping washing for a few days, a few weeks, or even a year did not "solve" the problem.
Try finding a gentle, natural shampoo, with minimal ingredients and try washing daily to see if it alleviates the problem. Not everyone's hair can handle stretching out washes, I am one of them.