Sorry to hear of your frustration.Could it be that you are not ovulating??.A day 21 blood-progestrone level may be able to tell.

I had the same problem though my periods were 6-9 weeks apart I was told to take a holiday and change my diet it worked I always get my period on time and my problems started when I was on the pill too though different pills may make different results

Are you anorexic, bulimic, or otherwise not eating healthy? When I was younger, I used to get my period twice a year- once in the winter, once in the summer. For like 2-3years. Then, suddently I was 115 and eating somewhat steadily if not a whole lot, and boom! I got it everymonth from then on. Well, close to anyway, I still skip months alot but rarely more than one in a row. It's directly related to how much/little i'm eating at the time however, as is how heavy/light it is.
So is there a chance you're not eating enough? Usually you don't get it often enough cuz you're not healthy enough for your body to expend that energy and those nutrients that get lost. Perhaps hyperthyroid, are you loseing massive wt for no reason, eating everything in sight, irritable and overactive sex drive? Those are all symptoms of hyperthyroidism. And that too would effect your period.
Just a few thoughts, good luck~
sarah