Eventually (!) I have my 2700VG minimally configured and working.

Now, for approx. 80% of the times when I make a further config change (such as adding an extra port forward) the 2700VG reboots and then gets stuck in a reboot loop which eventually zaps the configuration back to factory defaults. Infact, the only way I can stop the reboot loop is to remove all ethernet and adsl connections, wait for the reboot loop to stop and then plug the ethernet & adsl connections back in and then try to reload my last saved config (which takes me several attempts).

I am using f/w 2.8.2_131701.

Any ideas why my 2700VG is so unstable?

Its just yours, this is not an endemic issue. try emailing support <at> draytek.com to see if the have any advice.