Uploads causes ADSL PPPOE problems - Wrap board

[babstar]

Setup
PFSense: pfSense-1.2.2-Embedded.img
Hardware: PCEngines wrap1e103
WAN ADSL 1500/256 Kbps via Siemens Speedstream 4200 in full bridge mode.
PPPOE login: performed by PFSense
ISP IP: Dynamic

Problem: Uploading a large file appears to cause the PPPOE ADSL login to fail, causing PFSense to login again, killing the upload (and any downloads), and rolling the WAN IP address.
For example, trying to upload a 17mb file to amazon S3 gets to ~20% and then fails.  If I check the logs, my IP address has changes. Manually changing the MTU to 1492 or 1400 has no effect.

I have included a sanitized extract of system.log as an attachment.
Any help to further debug this problem appreciated.
system.log.txt

[babstar]

OK, I've done some more testing. 
I replaced the pfsense firewall with a tomato based router to do the PPPoE login (ADSL modem still in bridge mode) and I had much better success with large file uploads.  So this narrows it down to the PFsense as the source of PPPoE dropouts.
My link up speed is 256kbps, however with pfsense, once I initiate an upload, initially I get upto around 450kbps, then it drops back to the (expected) 222kbps, before the link dies.

I can't understand how uploading large files causes a PPPoE problems, when downloads don't.  Is there some PPPoE time-out mechanism that gets choked?  Any suggestions appreciated.

[jimp]

You might try to enable the traffic shaper, and set your upload bandwidth to something slightly less than your DSL sync speed (You can lost up to 13% due to various overheads). You can also try adjusting your ACK queues and such until it works better.

I'm using PPPoE on an ALIX and uploaded a bunch of files to one of my websites last night and it went at full speed without any issues. I have a WRAP for testing but I haven't tried it as my actual home router yet. I've got them both running 1.2.3-RC3 images though, not 1.2.2.