Please excuse my ignorance on this issue because I'm not a runner....but from what I do read, loading on carbs before exercise and racing are what the body runs on for efficiency and so that the body doesn't burn its own muscles.
so, add more carbs and see if that helps.
As well...have you had your vitamin D, B vitamins, potassium/sodium, calcium levels checked?
Are you hydrated enough throughout the day?
all those are reasons you can also have problems.
q