I have encountered similar, from time to time. In my case, the problem files are conversions to DV-AVI Type II, and it seems limited and random. The Duration is seen incorrectly, but I have never found out what is wrong - has to be something different in the file's header, but I cannot find the issue.
Quite by accident, I stumbled on a "fix," though it might not be viable for you. I found that PrElements could Import the problem file, with zero issues. If I just Import, drag to Timeline, and then Export to the exact same specs., DV-AVI Type II, that resulting file worked perfectly in PrPro, and all the time!
I have studied the two files, and cannot find any difference, even in a text editor, looking at the headers. Every utiltiy that I have run the two files through declares them identical, but they are not - one is perfect, where the other shows the wrong Duration in PrPro.
Wish that I had some sort of reason, or could point you to a value in the header, to change, but I am at a complete loss as to explain the issue.
As PrE fixes things for me, it might be possible that running the file through a conversion program (that is precisely what I am using PrE for - conversion), the Duration issue will be fixed?
Good luck, and though with slightly different files, you are not alone in this.
I can't help directly (don't use Camtasia) but some previous discussions of screen capture programs may help... not all related to Camtasia, but ideas for similar products may be helpful
Jing by Techsmith http://forums.adobe.com/message/3692768