I recently purchase a vigor 2830n plus and 2830vn plus and had major sync issues.

I have solved my issue, by reflashing my router via the methods posted to me by Draytek support as layed out below.

Hopefully this will help someone else.

Before starting this process please backup your vigor configuration.
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Try re flashing the firmware in TFTP mode.
Please use the .RST file, this will remove all of your configuration so you would need to set it up again.

Here are the instructions for a TFTP Reflash.

TFTP Reflash Procedure:

Make sure that the PC's IP address is set to:

IP address :192.168.1.X (X is any value from 10-254)
Subnet Mask :255.255.255.0
Gateway IP address :192.168.1.1

Download the firmware file from here:
http://www.draytek.com/index.php?option=com_k2&view=item&id=2448:vigor2830n-plus&Itemid=417&lang=en#downloads
Make sure that the firmware you download is not a beta version, as this will not include the .RST firmware file that is needed for the TFTP process.
Run the firmware upgrade utility found here:
http://www.draytek.co.uk/support/download/FirmwareUpgrade_362_Win.zip

Manually enter a Router IP setting of 192.168.1.1 and choose an *.RST file instead of *.all, make sure the Open dialog box is looking for *.* as a file type because it defaults to *.all.

Now, turn the router off, hold the reset button down while turning the router back on.
After doing this, the three lights on the left should be flashing together.
Then let go of the reset button.
Once this is done, click send on the firmware upgrade utility quickly before the router comes out of this mode, i.e. if the lights stop flashing together before you click "send", just do the process of rebooting with the reset button held in again.
If there is an error message of "Send Data Failed", this may be caused by any firewall software running on the computer, which would need to be disabled or have an exception made for the firmware to send normally.
If the firmware Reflash works correctly, the router should then operate normally.

Then re configure your connection to the ISP and continue to monitor the stability and performance of your connection.

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Boltymac