horizontal colored lines appearing after saving a PSD in PS

i recently bought a mac mini2, installed PS and LRC (all updated to the last version), and started working on my photos.

Now, randomly, when i finish editing a photo on PS, evertything looks fine.. until I save the photo.
The very nexy moment, the photo has a horizontal colored line, that appears in PS, LR, Finder preview...

the photo is a PSD produced by PS after i've worked on different raw files layered and then merged on a single layer. a workflow I've done for years, with no problems until now

In one occasion, the line has disappeared for no apparent reason.

I'm going crazy...