Unfortunately, pairing a "great" ADSL chipset with what I can only presume is a hopelessly underpowered CPU makes for a hopeless situation. As soon as you upload over around the 140kb/s mark, everything slows down for a few seconds (both uploads and downloads). Somewhere along the line, something in the TD-8840 just can't hope. By comparison, my old Open Networks 625 had no problems regularly pushing out over 300kb/s.
Looks like its time to shell out for a real modem. I'm still disappointed Open Networks folded. My 625 was rock solid.
In summary, TP-Link TD-8840: do not buy.