While sending the newsletter to over 800 users I received

While sending the newsletter to over 800 users I received this error (Error when updating DB) few times.

When that error occurs the newsletter sending will be paused as I couldn't see the progress bar moved (and #s too).
First time it gave me error it stopped at 1 out of 803 members. I couldn't send that newsletter any more as clicking send button and selecting the user group keep telling me that already sent.

Then, I created a new newsletter (what a pain to create duplicate newsletter and keep formatting consistent between them...a simple clone feature is much necessary for this plugin). I hit the send button and selected my user group. The plugin started sending the newsletter OK initially and displayed the error "Error when updating DB" after Newsletter was sent to "210 out of 803" members. Now I am stuck again.

Are there any log files I can see why this is happening..only change in last few days on this wordpress installation is installing "W3 Total Cache"