What model of camcorder is this clip from and what format and resolution is it?
Which version of Premiere Elements are you using and on what operating system?
Have you ensured that you have the latest version of Quicktime from Apple and that you have the latest driver for your video graphics card from the nVidia or ATI web site?
The clips are from my JVC GY-HD 200 and are 720P, but the other clips are 1080I. I'm using my IMac with Mountain Lion and Premiere Elements11. I have used these clips together before without a problem.
I ran across a forum for Premiere Pro about a similar problem when importing from AE that suggested applying the Crop effect to the clip without changing the size. I tried this and it seems to have worked, but I don't know why I had the problem to begin with.
Once you start mixing pro and consumer video formats, things can get a bit sketchy.
Your JVC shoots 1280x720 in HDV1 format in 24p, 25p, 30p, 50p, 60p and 60i, so I'm not sure specifically which frame rate you shot your video in. But I assume you captured your tape-based video over a FireWire connection and you used Premiere Elements to control the capture so that the video. Is that the case?
You also haven't said which version of the program you're using. But if you are using version 11 or 12, then it also makes a difference which video clip is the first one that appears on your timeline, since that will trigger the program to set up the project to match those specs.
So there are a number of factors that could be causing the problem. Though, like I've said, when it comes to loading pro video formats into Premiere Elements, things can get a bit sketchy.