Downshifting into 3rd for no reason

[DavieGravy]

Hello,

For some reason, if I'm cruising at about 60 or so, the car will downshift into 3rd for no reason, and without accelerating. It will then shift back into 4th and do it over again a few seconds later. If I cruise at a faster speed, like 70 or 80, it doesn't happen. Just seems like it's overly eager to downshift, even when I don't need it. Does anyone know what might cause this? The tranny is practically new.

Thanks in advance.