asterisk users - Oct 2005 - make sipura stop generating stale nonce. Device comes in and goes out every 1 minute

I am authenticating sipura device as a sip user to my asterisk server.
Things work fine and then suddenly asterisk console tells me:

Oct 26 23:09:17 WARNING[5096]: chan_sip.c:4826 check_auth: Stale nonce
received from 'Sipura1PSTN <sip:994@*******.dyndns.org>'

as soon as that happens if i try to call this sipura device extension
number it tells me:

   -- Executing Dial("IAX2/vikas@vikas/5", "SIP/994|20") in
new stack
    -- Called 994
    -- Got SIP response 410 "Gone" back from 59.xx.xxx.xxx
    -- SIP/994-cb94 is circuit-busy

and the call does not work.

The call starts to work again when on the asterisk console i see:
-- Registered SIP '994' at 59.xx.xxx.xx port 1249 expires 30

I have tried changing the following values in the sipura admin interface:
register retry interval -> 1 seconds.
register retry long term interval -> 30 seconds

But still it is generating the stale nonce.

How to make the sipura device stop generating the stale nonce,

Please help,

s