Zach, is the clock accurate on your receiver when this is happening? I have the same receiver, and lately during the morning hours, the receiver's clock will start to run slow by a few minutes, causing the LF timing to be screwed up. The clock fixes itself during the afternoon, or it can be fixed immediately by resetting the receiver.
On the other hand, I think I've seen a glitch like what you've described even when the clock is accurate, so maybe there's two separate glitches at hand here, and maybe the one you're seeing is the one not related to the clock. That would be especially true if the glitch you're seeing only happens at :18/:48. I don't think the clock would only happen to affect those two LFs.
