I'm having the same exact issue as you. It all started after I upgraded my firmware to Firmware Version 1.05.04 (2016-09-09)
Hardware Version A
Agent Version 2.1.0-b41
Before the upgrade, cameras were rock solid, never went offline or anything. Now they drop off the internet access non stop, I can access them on my local LAN but not through the cloud service. If I reboot them, it resolves it until the next day and I check again and they show offline. It has to be a bug in the firmware. Both my camera's are doing this.
Edit: i tried to downgrade the firmware on one of them and it went fine but now cloud services NEVER connect, even after multiple reboots, only works on the LAN.
Edit 2: On the device where I downgraded...it could not get it to connect to the cloud at all. So i reinstalled the latest firmware again and at least now I'm back connected to the cloud services. I'm sure it will drop off by morning ... WEIRD STUFF happening with these firmwares.