Rather than an LPFM transmitter, that location might also be in a straight line between two other stations. As a for instance, the math would work for 99.1 and 103.5. The difference between those two signals is 4.4, and 103.5 + 4.4 is 107.9. At that spot the other signals override the distant 107.9. Depending on your car radio you might get all 3 stations, or nothing. The reason you get it some days and not others is most likely weather related.
There is a spot in Jacksonville, FL where virtually every station in town comes in on 100.7.