<no config posted, falling back to "shoot from hip" mode>
This might be MTU problem. Try setting mss-clamp to 1452 for pppoe interface. When no pppoe present, set it to all ethernet interfaces
set firewall options mss-clamp interface-type pppoe
set firewall options mss-clamp mss 1452
You can do this from Gui , config-tree->firewall->options->mss-clamp
