Any idea what causes me to have seven mouth ulcers?

I've been getting groups of mouth ulcers every so often for about a year. My GP has sent me to an oral specialist, but they both are telling me that they can't find a cause. Any ideas?Any idea what causes me to have seven mouth ulcers?
I regularly suffer from mouth ulcers and find the best thing to do is change your toothbrush regularly, make mouthwashing a daly ritual and when you have a mouth full of ulcers try to brush your teeth after everything you eat if not brush then at least chew gum or at very least hold a breath mint over that spot. My dentist prescribed me some iodine mouthwash not the best flavour but certainly helped!!
If an oral specialist can't figure out why, do you really believe someone on yahoo is gonna know?
Sometimes they are caused by acidic foods. Try laying off soft drinks, tomatoes, anything acidic. Rinse your mouth with some water with a little bit of baking soda in it before you brush your teeth, too.
Do you consume foods that are high in acidity? (oranges, tomatoes, etc.) It's just a thought!
I don't mean this insultingly, but maybe oral sex with an infected person.