I used to have the exact same problem with my 67 Coronet. Before I did anything I adjusted the hood latch assembly so that I had to shut my hood a little harder (a soft slam) to close it and that did the trick. It's been about 4 years since I've had it happen.

I used to be able to shut my hood without slamming it. I would just close it and then press it down with my hands and it would latch. But if I hit a bump at highway speeds it would pop open to the secondary latch. Only happened twice but it was very scary when it happened.