- IPv6 is tried , but takes forever and has 100% package loss (frecuent).
- pinging IPv6 addresses works (very rare)
That's strange. A traceroute with booth working v6 and not working v6 would be helpful. Also a output of your routing table would be nice.
- pinging pure IPv6 addresses (ipv6.google.com) results in 'network not found', but pinging www.google.com results in a successfull IPv4 ping (very rare)
What is rare ? The successful v4 ping ? This is easy to explain. As far as I know there is only a AAAA record for ipv6.google.com. So when there are any v6 related problem you should not able to send packets to this address. www.google.com got booth an A and and AAAA record. I am not sure how ping decides which protocol to use, but in your case it seems like you v6 network is down (maybe no routing information), so it is using v4. You can force it to use a specific protocol with -4 oder -6 switch.