DNS CACHE POISONING? - Our Portal is redirecting to our first competition

[ponchovaldes () gmail com]

Hello guys, we have a social network that is getting stronger, but we are having an issue.

And the issue is that Sometimes... our page redirects to another Portal, actually the page that redirects is our first 
competition,here in Latino America, i know that they are causing that kind of mess.. so we thought in this.

- We know that our DNS server is ok, and havent been compromised,
- DNS cache poisoning
- Malware ?
- some kind of virus that the guys(bad) made. ( the other portal - social network-)

- Other soolution? sue them?

HElp guys.. this thing is taking out alot of users :(

thanks in advance!

Cheers from México