So, I finally figured this thing out.

It has to do with the RDS option AND the regional linking.

It seems that the station had regional linking available (99.1 FM was tied to 103.1 FM). Problem is, that "tying" wasn't valid for the area I was in, so I was getting a completely different station whenever my signal got a little weak.

Now here's the kicker. I had RDS turned off on the stereo (see Nav. manual p. 23). When RDS is off, you don't even have the option to configure the "Reg" and "PTY" settings ("Reg" being the Regional linking option). Regional linking is automatically on when RDS is off, and you cannot change this.

So I turn RDS on, and I check my current setting for regional linking. It's off. So, I drive around a little bit, and see if the stereo wants to mess with me again. Amazingly, it stays on 99.1 FM!

Cliff's notes:

- Regional linking is great, if you're traveling between the right areas. Not so much if you're not.
- Regional linking is automatically on if you have RDS off (you can't change the setting).
- You can turn regional linking on or off, only if you have RDS activated.

This is applicable for the stereo with nav. YMMV with non-nav (probably N/A).