Upgraded firmware from 3.3.7 to 3.6.0 last Friday. Then for 3 days my connection on WAN2 (Bethere ADSL) would start to deteriorate until I restarted it. And then after a random amount of time it would deteriorate again. The 2920 is set to reboot at 3am every morning as well. I have been using a Netgear DG834GT in modem mode successfully for 6+ months.

Here are the symptoms:
Packet loss from LAN -> WAN would be significant
Latency was doubled and erratic, LAN -> WAN
Thoughput was less than 20%, again LAN -> WAN

However, here is the odd thing. Thinkbroadbands ping monitor and Bethere could see not packet loss or high latency. So from WAN -> ROUTER all was fine.

I have gone back to 3.3.7 and the last 3 days have been fine again.

WAN1, using VM Superhub in Modem only mode is fine.

Problem with 3.6.0?

Just tried the 3.6.3 and this firmware has the same issue.

There is a beta here but not sure what it fixes
ftp://ftp.draytek.pl/Seria_2920/Firmware/beta/
But did you master reset before you go to 3.6.0 and do all settings from the begin? Because from that version was ipv6 ready so a general clean with rst file is a must entering the 3.6.0 and upwards
When had my 2920n with 3.6.0 (did not tried any other upwards firmwares because i connect back my 2850n) had no issues with my VM 100mb
Are you with VM 120mb? and you getting full speed?

Thanks.

Yes - tried master reset on 3.6.0 and 3.6.3 and imported config.

The VM connection is fine. Its the ADSL (Bethere) connection that has the issue. I checked every setting and cannot see anything different. Turned off IPv6 as well.

Don't know what to think really but here some points to check out

Maybe try change to different cable from BE modem to wan 1 and see if still there is problem
Then maybe from the lan of the 2920 check cable again (going to pc)
Disconnect cables and see if you connect just wireless if the issue still there
Check the NIC card of pc (its settings)
There is also one option to try at the WAN >> General Setup ...click wan 1 (assume that is where the BE is been set up) and check or change the "Physical Type"
Also at same page should be at always on for load balance and fail over (Enable YES)
Also (not know what the settings are for BE but is in bridged mode the BE modem and then set up as pppoe at the 2920n?

EDIT & important

dont think but is worth it check and firewall if blocking lan to wan etc(default should not be problem but if you use previous config may affecting so try disable it and see if the problem still there
(draytek recommends use of rst file and not use config from previous firmwares)

Thanks.

Used RST and started from scratch and just put the bare minimum in to connect to BE and still same problem. Poor latency and horrendous throughput.

Tried your suggestions and no difference.

Went back to 3.3.7 and perfectly fine. 3.6 onwards is definitely broken.