If your setup works when you leave some of the settings at 1500 and others at 1492, it's possible that the devices set to 1500 are auto-detecting the other devices and resetting MTU to 1492 whenever you transfer data to/from the Internet through your PPPoE connection. I suppose that the only way to be sure is to run the test again.
Earthlink generally doesn't support PPPoA, although Covad/Megapath [which was a partner of Earthlink for a long time] does. There were, if I recall correctly, a few regulars in this forum who had Earthlink DSL and were connected via PPPoA. I'm not sure if that's the case any longer.