I used to suffer from this too, but found out it might be due to a lack of iron, so I started taking iron supplements (easily available) and the problem all but went, I still get it from time to time, but believe me, its nothing like it used to be. Hope it works for you.
Absolutely agree with Cappyandole. Have blood tests. If you're a bit anaemic, like it turned out I am, get iron tablets prescribed and, hey presto!, you could be cured like me. Fantastic! :O)
just one more thing. look at all these ads around the answer bank site, to the right and underneath where i'm typing. i hate these people who make money out of the suffering of others. reckon they've found a miracle cure and charge you a fortune for it..!
take iron tablets (once checked out by your doc). don't waste your money on ebooks and quack medicines!