I have been experiencing video lag in Camtasia 5.  It would seeming ly happen without rhyme or reason and was a progressively increasing lag where I could hear my voice talking about things I had not done in the video yet.  There are a few remedies that seem to fix the problem or at least help, and I will list them here.

1) If the audio is thrown off after producing the video but looks fine when reviewing it before it is produced, make sure you are including an index during production.  The option can be seen in th ecustom production wizard:

2) It might be caused by a memory leak of some sort.  Rebooting before each video and recording short segments could aleviate the problem.

3) While recording the video, have all the applications you will need open already.  I noticed that the lagging recordings coincided with times that I opened Windows Explorer during a video recording session.

Note:  From what I have read, these problems cropped up in Camtasia 5, so this might not be a problem at all in version 4.

I hope this helps someone out!