I had this happen with one of my older phones. Is it possible that it is only when a certain person or certain group of people calls? I couldn't figure out why this was happening with my old Nokia, then I realized that it only happened when I was receiving calls from all of the people in a certain group on the phone. I went into the group settings and noticed that that group was set to only vibrate when I got a call. I changed this and everything went back to normal. I hope this helps!