There are two basic causes for seizures. The first type is metabolic. Things such as low blood sugar (glucose), or an electrolyte imbalance (sodium, calcium or potassium). Both of these are possible but usually occur only under extreme physical stress or as a result of another serious illness. Blood testing can be done to help identify these causes or in the case of low blood sugar the history is sometimes helpful. For example if they occur after long periods of extreme physical activity.
The second cause is seizures. These do occur in a certain percentage of dogs and usually begin to occur when dogs are a year to two old. The seizures can range for mild to very severe and over time a pattern usually develops for both frequency and severity. There are several drugs that are used to help control seizures. Phenobarbitol was mentioned and there are several others. These drugs raise the seizure threshold which often reduces the number and somtimes the severity of seizures. There is no cure for this type of seizure but there is hope for sucessful management and a normal life including hunting.
Many dogs sense they are going to have a seizure and certain behaviors can be identified which helps predict seizure activity but my experience with hunting breeds is when they are hunting they forget all else and give little warning. As far as giving chocolate to a seizing dog. Be careful. Most seizures are very short in duration and probably are over before the chocolate does anything. It just appears there is a relationship between the sugar and the end of the seizure. Second dogs can and do bite during seizures as they have little control of their behavior. Third chocolate is toxic to dogs although in my experience small amounts in big dogs has not been a big problem. I have seen small breeds die from chocolate toxicity. Karo syrup or another sugar dense syrup would be a better choice although hard to carry in the field.
In addition to seeing dogs in the clinic I hunted with a friend with a german shorthair that often seized when he went on point. I medicated him and he improved somewhat but still occaisonally seizes when he gets around birds. Fortunately for him there are not many grouse around here now. \
good luck.