I have a Linksys WRT54G (v4) flashed with the newest version of Tomato/MLPPP.
I have a SpeedTouch 516 and a SpeedTouch 546.
I have 2 phone lines.
When I connect each modem individually directly to the computer and then connect via PPPoE I get a regular connection with regular (though unsatisfying) results.
When I connect everything to the router I get a sawtooth graph of connectivity. It will connect for about 30 seconds and then disconnect for the same amount of time. It's driving me nuts trying to figure this out. I've included some small helpful graphics showing the upload graphs from Vuze to illustrate.
I've reflashed the router a couple of times and reset the modems individually a number of times nearing infinity.
I'm going to try it one more time by resetting everything to factory defaults (again), reboot everything and see if I can at least get something resembling a working, stable connection.