Raul,
When in Nav with the A/P on, an active glide slope will drop to the bottom. But only on the Nav mode. For me, as stated, I correct the problem by moving the selector to Nav/Loc, and the GS comes right back up again.  From there you can move the selector anywhere you want (other than back to Nav), or turn the A/P off, and the GS will function properly. I just tested it again and with the same results.  This is how it works for me anyway.