what foods enhance mental performance and delay fatigue?

Generally, if it's food related it's related to the type of carbs you are eating and how much fat is in your diet. If it's happening before lunch it could be because your breakfast didn't have enough complex carbs in it. If directly after lunch your lunch was probably to heavy and your body is focusing it's energy on digestion. Low GI (glycemic index) foods are the best.

Try these websites about fatigue at work and nurishing the brain

I would suppose these can be food stuffs, gymnast and weight lifters eat or drink honey. Another person suggested chocolate, true, that works too, but with chocolate you want to make sure you eat a good breakfast, this is something I was going to mention to you. Your description of your job would suggest that you eat something before you leave the house, if you haven't, take the time and try it for a few days. You will see and feel the difference......LATER

Drink lots of fluids non caffeine as dehydration is the major cause of fatigue in the afternoon. Eat high protein snacks.

try snacking on something healthly instead. I had the same problem but I start snacking on sunflower seeds and nuts and not having a real heavy lunch and it worked!

Chocolate everytime