Is this in all files, or just one? What's the source of the file?

Are you setting the tempo using the playback function of a Tempo expression, or are you setting the temp in the little box as part of the playback controls at the top of the screen?

Is you audio setup configured to sample at 44.1 (standard CD sampling, or at a higher rate: 48 or 96)?

Those are the first things I would check.

What's really strange, I've discovered, is that I will have it set at a certain tempo and on playback, once it passes a certain point, the music will suddenly speed way up or slow way down.  I was not having this problem before and did not change anything that I'm aware of.

I'm setting the tempo in the little box as part of the playback controls.  Not sure how to check the audio setup referenced above.

How do I determine the source of the file (to answer Mike's question)?

You might try clearing the MIDI data. (Select the score, right click > Clear Selected Items... > MIDI data.)

Did you create the file? Was it from a scan? Did someone else send it to you?

I created the file.

That box on the playback control is really only for a single tempo. Use expressions to set changing tempi throughout the piece. You can create tempo markings at whatever speed you want. Expression Selection > Tempo Marks > Create Tempo Mark