d-link DI-614 problem with timeouts

the following is a good discription of my probelm.
I have problems with extremely slow and/or timeouts on my home LAN
during FTP uploading (downloading is fine).

My set-up:

ISP:ADSL --> DSL Modem (172.16.x.x WAN, 192.168.x.x LAN) --> DI-614+
Router --

I start an FTP session to a site, it starts uploading and then slows
down to the point of dropping and timing out. I have tried different
FTP programs. I have also tried using DMZ and Virtual Server
settings.

If I remove the DLink DI-614+ Router from the picture and connect to
2-wire modem, everything is fine.

If anyone has some suggestions I would appreciate it.
thanks
john