I was having this problem for quite a while, until I consulted a homeopath who did a food intolerance test. The report came back with 2 lists of foods that I am severely and moderately intolerant to. It ended up that I was eating many foods - which would otherwise be very healthy - but that I was intolerant to. Things like strawberries, oranges, grapes, salmon, yogurt and many more. When I eliminated the foods on the severe list - I felt alot better.
I believe the company performing the scan is Gemoscan - perhaps if you look up their site, you will get more info than I have provided.
Good luck.
Yve