Template Upgrade Problem

I'm having problems upgrading my template for this blog from a classic template ("Rounders 3" template from early 2005) to a new template.  I have multiple classic template blogs from early 2005 and multiple new blogs created with the new templates.  I want to upgrade all my old templates to the new ones.  My first attempt has some problems.

When I upgraded to the new template, I entered the template designer to choose a template.  Everything was very slow in loading, and my preview is blank, except for the rightmost 1/2" and bottom 1/2".  See photo above.  Sometimes, even the part of the preview that shows up goes blank.  There are other problems too:

• Changing any setting in the template designer is very slow.
• The cursor flashes very rapidly between a normal cursor and that rotating circle (MS Waiting!)
• The blog page loads very slow from any computer, even though this blog is more simple (fewer links and gadgets) than my other new template blogs.  Same profile photo, etc.
• My other new blogs with the latest templates don't have any of these problems.
• I have two computers, one with IE8.0 and the other with Firefox 3.6.12.
• The above photo is from the IE8.0 computer.  The Firefox computer gives a normal template design preview, but everything is just as slow with changing settings, etc.
• I've changed my template many times as well (Simple, Awesome, etc.), but there is no change in either the template preview or the sluggishness of changing settings.
• Occasionally, when going into the template designer, the preview will show the whole blog page correctly for a fraction of a second, then change to what's shown in the photo above, then it may completely go blank.

Any help is appreciated in advance.  Thanks!