Mindless,
About the only suggestion that I might add is to check on the condition of your (physical) line. My solution to this in the past was that I simply ran a new dedicated length of nice, pristine CAT5 all the way from the telco demark on the back of my house right up to the "computer area" <-It can hardly be called a computer desk now... it's more of a region of the house.

and used one pair for my voice line and one for my data line. I consistently gained a few Kbps of connect speed immediately and now I'm darn sure that if there's connection problems, it's not me, but rather the telco or the ISP.
Other than that, I'll make the offer of an ISA Zoom Telephonics 28.8 with a Rockwell chipset that you can use. Keep it for testing or just well.... keep it if you like. Let me know.
-Brian