Problem selecting Plot Style Table (Selects different table from the list)

I've recently encountered a problem with Autocad 2005 when I try to select a plot style table for plotting a drawing file. What happens is that I pick one listed table and a different listed table shows up in the dialogue box. It seems to be shifting a consistent number of tables each time; that is, I'll select the 10th listed table down the list and it returns with the 12th listed table.
I have reset the order of listed items so that all the .ctb files, in this case, are grouped together. That precludes the situation where the apparent "shift" is due to the list showing similarly named files of different extensions being adjacent to one another in the displayed list.
Has anyone else encountered this situation? How do I resolve it?