I've always been pretty careful about keeping foods with any additives out of my diet and my daughter's when she was little. But I have a few times eaten such foods as a guest in others' homes. I feel the effects almost instantly – my symptoms vary somewhat with the chemical, but can include tiredness, nervousness, confusion, loss of "words," fast pulse, anxiety, shortness of breath, headache, and a less-defined feeling of being unwell.
This study showed a clear link between certain food colors and a common preservative and behavior in children: http://www.time.com/time/health/article/0,8599,###-###-##...
Another cause of behavioral mayhem can be toxic chemicals in the home environment. We live in a rather alarming toxic stew that contains many chemicals that didn't even exist commercially a few decades ago. I have developed severe chemical sensitivities, and get both physical and emotional symptoms to exposures to perfumed toiletries, home cleaning products, fabric softeners and air "fresheners."
In group testing situations, I have watched children go from contentedly coloring to bouncing off walls, screaming, crying, or being impossibly stubborn just minutes after having drops of some dilute solution squirted under their tongues.
It would be worth checking out. You can try sealing all suspicious products in plastic bags and using free and clear detergent, baking soda or vinegar for most cleaning for the next few weeks, and watch for any improvements. If reintroducing the products back into the home then results in worse behavior, you'll have a possible solution to work on.
I know this sounds like a lot of change, but it's really not that hard to try. Most of the household products and toiletries commonly used are really not needed, and they are expensive. We believe we need them because the advertising is so compelling. But I've used very little for cleaning in my home besides baking soda, white vinegar, borax, hydrogen peroxide, and scent-free detergent for over 20 years. My house always smells clean and pleasant – and visitors often remark that the atmosphere is "calming." I think their bodies are noticing the lack of toxins in the air.