576 is pretty low. Try pinging with no fagment flag:
ping -f -l <lenght><wan-gateway ip="">(windows syntax)
your mtu should be the highest length you are able to ping +28 (overhead). A typical mtu value for DSL should be 1492 but your's seem to be lower as this is the default value pfsense will use for pppoe. Try to find out your maximum with ping.</wan-gateway></lenght>