Web Interface on the modem.
Read this thread from begining to end, there has been a lot of info passed on to you already on where to start looking.
http://www.dslreports.com/faq/6323 - Step # 2
Look for either diagnostics and or signal information.
192.168.1.1 may also work?
If your modem is in sync you can also call the DSL provider obtain this info and they should provide it to you.
We need to know the following:
What data rates are you paying for and what is the modem currently running?
What is the up and down signal attenuation?
What is the up and down signal margin?
What is the up and down transmit power level?
What path scheme is being used, Fast or Interleaved?
If you connect the modem directly to the NID does the DSL sync right away, if so, you may have wiring issue in the house?
If you determine the problem is on the phone company side, I would be asking for service credit until the problem is fixed.
JamesO