I, too, have about one longer-than-average cycle per year. For me, I have not found a pattern, but the longer cycles are not more than about 8-10 days off. What is your average cycle length? Have you noticed anything spceific about the long cycles? I wonder if they are related to an annual event or more random. For ex, maybe your ovulation is really delayed every year as you gear up for spending a week with extended family, etc.