FYI - in case anyone has the same issues:

I'm running a 2710Vn which acts as my VOIP system back to Draytel (who have provided me with an excellent service BTW).

As well as the 2710Vn connecting back to Draytel - I also have a number of Softphones connecting to multiple Draytel accounts. Frequently, these softphones sit behind the 2710Vn - they are configured to connect to the NAT address (with the 2710 connected direct).

Further to this, I used a Vodafone Sure Signal device as it's the only way to get good 3G signal for my mobile in my house.

Under 3.3.2_211801 I had to disable DoS functionality in order to get the Sure Signal box up and running with any stability. That's fine. Other than that, this code just runs without any issues.

The moment I upgraded to 3.3.5.1 or even 3.3.6.x (from the .com site) - the Sure Signal box repeatedly keels over and disconnects - and any softphone clients sitting on the LAN behind the router also frequently disconnect and fail to reconnect.

Reverting the firmware back to 3.3.2_211801 instantly fixed the problem and both Sure Signal and softphone clients connect with 100% stability.

Basically, I think the code for 3.3.5.1 and 3.3.6 appear to interfere with session based connectivity to the point where it's completely unreliable - and at least for me, I cannot advance to this code until these problems are identified and resolved.