Hi. My name is*****'m a camera repair technician with almost 30 years in the business and I'll be glad to help you with your question today.
All you need to do is set the camera to TTL in the flash control portion in the menu. There may even be a setting that turns off the pre-flash or flash-focus assistance. Be sure the flash itself is set to TTL or ETTL as well. I've never actually seen a flash that has a setting to produce the effect you're describing. Also, I've repaired many Nikon cameras that ha produced random firing. They had defective hot shoes an needed to be replaced. Is the problem you describe random or consistently spaced
Hi Colleen. does the flash fire on its own when not attached to the camera?