I doubt the wavelength is a factor there; depending on the circumstances it could be anything from atmospheric waves to something in your car causing intermittent interference.
The Earth’s ionosphere exists in several layers. During the day, solar radiation ionizes gas deeper into the ionosphere causing a layer that doesn’t usefully refract most radio waves; you can reach beyond the horizon on some of the higher HF bands, but down in the MF, you’ve basically got ground wave. At night, without the sun around to cook the atmosphere, that lower level dissipates, revealing a higher ever-present layer, and the geometry is right to refract signals for hundreds or even thousands of miles.
Skywave propagation can be really fun to play with.