FPS dropping to 30 while v-synced on a 60hz monitor.

After I updated my PC last night with the the most recent Windows update, I noticed that my game will now randomly drop to 30 FPS for about a minute or two, then go back to the normal 60 FPS, while using v-sync. For now I've just disabled v-sync until it's looked into. Without v-sync I get a consistent 200+ FPS, but I prefer v-sync.