First, I need to state that I've never dealt with pppoe connections in linux ever before. The last time I had a pppoe connection was before I knew what linux was.
However, there are a couple things we can try.
1) I'd like to see what your 'ifconfig -a' looks like after a fresh reboot.
2) Next, I'd like to see what your 'ifconfig -a' looks like after you've attempted to start the pppoe interface (i.e. pppoe-start).
3) The output from your routing table before and after would also be nice (route -n)
What I'm thinking is that perhaps you are ending up with what your system sees as two possible network connections (e.g. eth0 and pppoe) and your system is attempting to use the more common eth0 connection. It may also just be a routing issue.
Last, perhaps changing the subject line of your original to something more specific will gain the attention of someone who has a similar internet connection as your. A subject like "Help setting up PPPOE internet connection" would probably help others quickly identify the specific help needed.