It seems like IE's preference for borders might be related to a collapsing margin issue, especially if the border is preventing margins from collapsing. However, upon closer inspection, there doesn't appear to be any noticeable margins in your show/hide content.
Could it possibly be due to the padding instead? The 10px padding on the .trial div could be causing the jump when rolled up or down. It's possible that IE7 is struggling to handle the 'rolling up' of the padding, thus hiding it all at once and causing the abrupt movement.
You may want to experiment with adjusting the padding to see if changing it to 20px at the bottom of the .intro div instead of 10px on both .trial and .intro resolves the issue.
(Disclaimer: I don't have access to IE7 for testing purposes, so I may not fully understand the jump you're referring to. Feel free to correct me if I'm completely off base.)