MTU of 1492 seems to be hardwired somehow

Yeah, the pppoe spec is for 1492 so you have to make everything else match or some of your packets misteriously disapear. Try lowering the MTU on the NIC in one of your client machines that is having issues to something under 1492 and see if that helps. If it works then you'll want to figure out how to get your NAT software to fragment packets if necessary to keep them under that size.

-Mike