What is stock voltage? 1.65 volts would be dangerously high on modern processors but those old ones like you have used more volts.
Also, how much at time are you increasing the frequency? Try small increments at a time like going from 800 to 810 to start with.
Finally, you would likely need to lower the starting frequency of your RAM to give it some overhead. The frequency of the RAM will increase along with the frequency of the CPU and the RAM will likely not tolerate much of an increase over stock frequency.