The reason it happens is because you have short circuted the modem to the hub and the pppoe protocol or software have trouble keeping things in sync because of timeouts that were generated by the network collisions. The solution is to seperate the modem network from the home network so no collisions will happen. However, the CLAMPMSS=no could also help since it can reduce overheads and thus lower timeouts (if any)though i didn't check that. other things can help like increasing timeouts, hanging synchronization options but it would be cheaper by time/money to just buy another ethernet card and be done with it.
1. vi /etc/sysconfig/network-scripts/ifcfg-ppp0
..
CLAMPMSS=no
..
2. /sbin/iptables -t mangle -A FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu
