Follow Grum's suggestion and see what service speed you are on.
If it's the 64k service I would suggest switching modem off for as long as you can bear to be without it and switching it back on again + see if that gets you the proper DHCP and config file - what are the last 4 digits of your IP address at the moment, are they by any chance .248 or higher? (Not 100% sure it's 248 but is somewhere around there)
