You're probably thinking of the Motorola Surfboard, the SA is a Cisco modem (replaced the Ambit's) and it was only superseded by the Hub/Super Hub.
As long as the coax cable used is R6(R59 if you have to)/HD100 then you shouldn't have any problems. As PistolPete said, it's probably an issue elsewhere.
Does it appear to be capped at 300kb/s or does it fluctuate throughout the day?
Best bet is to get onto the cableforums.co.uk forums and ask on there, plenty of VM/NTL/Telewest engineers that'll be able to help you.