Under Edit> Project Settings> General do the video and audio parameter match with the actual clip parameters (you can check with a utility like GSpot or MediaInfo) ? PRE does not always get it right when you first add your clip.
In Expert mode is there an orange line running the length of your clip ? This is another sign that the project setting is wrong.
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Did you reinstall Premiere Elements 11 as part of an uninstall/reinstall because the program was not working correctly? If so, what problems were you having before the uninstall/reinstall?
Were these same AVCHD files importing and playing back OK on Premiere Elements 11 before the uninstall/reinstall?
Please review the following blog post on problems that can be encountered by letting the program set the project preset for you automatically.
Depending on your details, you may have to do a manual set of the project preset as described in the How To section of the link.
But, let us explore the background information on your issue to make sure that we are not taking anything for granted.