Ways to limit idiosyncratic fainting/seizures?

2 different things. Seizures are electrical events in the brain interfering with normal brain functions including muscle tone and consciousness. Causes of seizures may be undeterminable at times, hence "idiopathic". Fainting on the other hand results from interruption or reduction of blood supply to the brain. If the cause of this is undeterminable, then this is also idiosyncratic, i.e. Not determinable in your case.