Thanks kilokat! I hope someone can confirm whether Sparta is the culprit. If so, I will contact the station and let them know.
I have reported off-frequency operation at least three times previously to various station owners. In every single case, they were very cooperative and corrected the problem. That's a win-win situation because the interference goes away, their own listeners get a better sounding signal, and they avoid an FCC citation. I have found that if you're polite and sound like you know what you're talking about, they will usually pay attention.