asterisk users - Jul 2003 - -- Got SIP response 481 "Invalid CSeq Number" back from 216.52.153.207

hi ..

I have a very simple asterisk system which is not working properly ..
basically I have an asterisk box, with a PRI-E1 card accepting calls,
and immediately forwarding them, via SIP, to 723@216.52.153.207
which is a commercial calling gateway.

Sometimes it works, but very often I get this message :
 -- Got SIP response 481 "Invalid CSeq Number" back from
216.52.153.207
and the call is dropped.
It seems random.

does anyone have any idea how to control this behavior ?

thanks
Dave