Check out some of the other speed testing sites listed in the Rource forum:
Resource LibraryIs this the modem your ISP have you to use?
One thing I highly recommend checking is the ISP Cable lines going from the outside in to the house and to the modem. Make thats in good condition. If there are any TV line splitters on it, these will lower the signal going to the modem, if a splitter is needed, use online one good quality one. Have the ISP make sure the signal going to the modem is correct.
I would highly recommend asking your ISP if they support Motorola SB 6 series stand alone modems. These are good modems, I have tested 3 models out. All work well.