i have stuck with 3.3.2_RC5 and not had any problems with. You might have a faulty unit.

You tried moving it somewhere else in case it’s getting interference from something?

I also did a fully reset of the router then entered all my details back in after flashing with above firmware.
Not had the problem again since then

Ive had 3.3.3_RC3 beta from the Australian Draytek site for the last few days with no issues.

http://www.draytek.com.au/downloads/Vigor2820.php

Give it a go!