Can you be more descriptive about the artifacts you are getting?
Are you using the latest version of Toast (10.0.2)?
Is your OS up to date? 10.5?
Beyond these questions, the only other thing I can think of is to check your encoding settings in the Quicktime dialog. You can use
http://mediainfo.sourceforge.net/en to check the specs on the .MTS Source File to make sure you are setting your encodes properly. I don't use the Private folder for toast, I just drag and drop the files straight in.