I have seen this once years before in a -900. I wrote it up after the landing and followed up with maintenance. No fault was ever found and we also came to believe it was due to a false glide slope. In level flight approaching the marker the G/S needle had just wiggled alive and the aircraft went off to the races in a climb. It can be easily controled by selecting AP off, but the flying pilot was a brand new ab initio FO. We climbed about 500 feet before I got him to release the yoke and let me get it back to altitude. Stuff happens. That is why they still put us in the pointy end.