Bad connection spa3000+speedtouch 580

Hi all

I'm a user of a spa3000 for a year now in combi with a linksys wrt54g
router, and it is working fine here.
But i installed a spa3000 last week at my sister's place, and the connection
is bad. She has a speedtouch 580 router/modem.
Is this a problem with this router/modem and what can i do about it.
Strange is that when i called to the US yesterday, they had no complains
about the sound, but on my side the sound was with a stutter.

thx for your help