When the connection are down, I can't ping the Gateway.
I can't login with winbox, too.
I only see, that the pppoe connection are stable and it sends information but not receive.
The DNS can't ping, too.
But when all works fine, I can ping always.
It's a amusing thing.
Now I have reset the MT and reconfigurated without backup.
I hope that the problem is resolve
Are you by any chance using windows xp service-pack 2? Try turning off all firewalls. Other than that, the problem is unlikely to be pppoe, it's more likely the transport layer or your client.
Read up on bridging and pppoe. Read how they work together.
You need to know what you are doing. You would possibly benefit the simplicity of a statically routed network instead of a bridged pppoe network. it's often easier to understand and get right.