Henry,
a lot of people have food sensitivities that can cause them to get congested. For some people, beer and wine will get them congested and put them off their game. This happens to me, so I have to be pretty careful, especially in the week before a race.
Other people are affected by things like milk products, nuts, mushrooms, corn, gluten/wheat products.
One way to check if it's a food tolerance issue is to ask yourself: is there a food that I think I absolutely couldn't live without? If there's a food that you think you NEED to have pretty near every day (maybe coffee? maybe chocolate?), there's a good chance you're actually allergic to that food, and the craving you feel is related to the allergy.
If there's a food like that in your diet, try to kick it for a couple of weeks (which will be hard) and see if you feel any better.