I bought these strings to use on a 4-string guitar of same scale length. It will not stay in tune. I had the same problem with a Musicmakers classical guitar. I gave that guitar to a friend. He restrung it and says it works great.
If an instrument won't stay in tune - it is almost never the fault of the strings. These strings work fine. They do take several tunings to stabilize. If they don't hold pitch after several tunings, I'd look at the way they are attached.