I will say upfront that we've done some modifications to the way gradebooks are displayed in some cases, but I am not sure what could have caused this issue. Looking into the HTML with Firebug, I changed the rowspan for one of the glitched cells, where it was "3", and changed it to "1", and part of the problem was resolved. So, the issue appears to be related to the determining the number of rows the cells that display the glitch should span.
Can anyone offer some possible insight into how to resolve this issue? Thanks!