Artisan, I know that I get heart palpitations if I have anything with aspartame (nutrasweet) in it, which is apparently a fairly common symptom of intolerance to this artificial sweetener. If you read the ingredients you may find something like this lurking in the bread and cereals you've got. Of course, it could be the grain, so trying gluten free is definitely not a bad idea. Either way, I know those palpitations do not feel pleasant!

Hugs and butterflies!
~T~