Ogo,
First try doing it at 1.85v and see if it runs. If it does not, I would definitely suspect that your chip is not capable of running any faster.
Granted that many other people have had more success, you must keep in mind that a LOT of people are experiencing the same barrier that you are. Another thing to think about is that seeing that your Duron came unlocked (yes, I did read your message), it came from an early batch. The earlier batches most likely are lower yield, thus resulting in a lower max OC.