Several other handlers I have spoken to have encountered a similar situation and believe they have just worked their dogs too hard. Some have solved the problem by feeding a Mars bar to help increase the dog’s sugar level. Have you had any experience of this?
I have used Mars bars to deliver a quick energy boost to spaniels in field trials when, after an exhausting first run, an experienced dog may have lost that extra edge required to deliver the same performance in its second run.
The fits your dog is having are probably caused by hypoglycaemia (low blood glucose), produced by an excessive amount of energy expenditure during work and must be treated quickly.
Honey is the best source of glucose, but Mars bars are far easier to carry.
Prevention is better than subsequent treatment, so the odd treat during the day (or at lunchtime) should be beneficial.
However, beware that too much chocolate can cause a dog serious harm.