[lathspell]

I agree with TolkienGurl. It was the Ring at work. It used Frodo, and tortured him while doing so. It was the will of the Ring and of Sauron. Frodo was to weak for such a contest.
The Stabbing in 'A knife in the Dark' is, I think, in no way related to the almost disastrous ending on Mount Doom.
I believe the stabbing was a way to win the Ring back. They did it that way and not in an open attack because there were only five of them, and, I guess, because they were still not in open war with the other lands, and therefore the Nazgul were still a secret known to few. Speed and secrecy were dreadfully important.
Killing Frodo was their purpose. When they knew that he survived it (hardily), and they were faster as ever moving towards Rivendell, they made plans of waylaying them. And they almost succeeded, if not for Glorfindel. If he had not chaised the Ringwraiths from the Last Bridge, I believe Strider could not save the Ring there.

This is how I see things.

greetings,
lathspell