My border collie puppy is falling down a lot. If he bends his head down his feet will go over his head. He will get an arch in his back and that's when all this happens. He is playful some of the time. He has vomited a couple times and has diarrhea but I also have changed dog food.

He sick!

Time for a vet check.
Diarreah, vomiting and being unbalanced is serious and needs to be seen to immediately.

I would wonder if these 'spells' might not be seizures, in fact. Would explain the sudden appearance, and probably the vomiting and diarrhea. If they are, your vet may not be able to accurately diagnose them as such without extensive (or is that expensive?) testing, repeated episodes may be so strongly suggestive that a medical approach may be tried, looking for effect on controlling the future appearance of them.

If this pup immediately gets back up after falling, this could be something besides a seizure, and may involve an x-ray of his neck to rule out a problem there. Actually, your vet is the only one who has a chance to accurately diagnose the problem. We're just making guesses which are essentially irrelevant... right or wrong, you need the vet.