Outside bandwith good, speed coming out of router extremely slow

Hi,

I have the 18mbps internet package (DSL with a BW210-700).  I have had multiple problems with my internet - have had technicians out 5 times now.  All of the outside components have been replaced - at one point they changed which wire pair I was on, etc.  A few weeks ago, I noticed that I could not watch any sort of streaming video, including lower bandwidth stuff like YouTube.  The tech came out yesterday and tested all of the outside line stats - they looked great.  He replaced my router and suddenly I was getting 15 mbps again.  But, it only lasted a few minutes and went back down to around 2mbps.

The speed itself has fluctuated greatly.  It will occasionally get upwards of 12mbps, but most of the time is less than 5.  I did an overnight speed test and it spiked up a few times but was low most of the time.  I had kept in touch with the tech and his supervisor and they both performed additional external tests and got good speed - around 16.5.

I have tried removing all other connections from the BW210-700.  I have tried wired and wireless.  I get no improvements.  But, to further muddy the picture, when I log directly into the router and do a speed test, it comes back good with like 16.5.  How can the router itself be getting 16.5 between it and the backbone, but inside the house I'm getting less than 5 even wired?  Is there some sort of throttling software or interference that can cause that?  I have not changed the router's location or any other major network configuration in my house.  This happens with a single laptop connected and NOTHING else.

I need help because the local techs do not know what to try.  Thanks, Brian