Do yourself a favour and check the firmware version of the IPU card.
Also, which card is it? BIPU, LIPU, or MIPU?
Also upgrade the firmware on the sets.
Also download pingplotter. Run it for a while when you are having problems. It will show you which router hop is being the bad boy (saved my butt a bunch of times).
I had an LIPU that was causing me grief, I thought it was internet related, ran pingplotter, connection pretty good, upgraded the firmware and presto, problem gone!
Dave
