Also, when right clicking on the study and then "Details," I notice that the number of modes = 15, yet I have the "Number of Frequencies" button under Frequency Options turned off, and as I mentioned above, the software actually calculates 173 natural frequencies.
What version and service pack are you currently using?
Can you post pictures of your study properties? Better yet, setup a simple block with the same settings and test that. Is the issue still there or does it go away? Post that model for others to take a look at.
Other things that would be helpful to check are your results options and what happens if you choose options like defining the number of freqs vs upper bound. For example, test the tutorial example.