Bob,
Largely a shot in the dark here... did you use the new RJ-11 cable that came with the new router, or did you use the old one?
The reason I'm asking is, that I know some old 56k modem cables were wired differently to other ones, and I'm wondering if there's something similar going on here.
It just seems slightly odd to me (and probably you too!) that two new routers don't work, yet the Etec seems to work with no problem.
Like Kitz says, the DG834(G) has a very good reputation with long lines. There's been several instances on other forums about people noticing success with this. Whilst I don't have a particularly long line myself, my DG834G is holding a steady sync at 8Mb whilst the Noise Margin drops to 0dB (and on occasion -1!). Most routers would give up the ghost when it dropped to about 3dB, and they would drop and reconnect at a lower speed.
Sorry if you've already tried the cable, but it's the only thing I can think of