I had an interesting experiance flying the ILS into Cambridge yesterday. GS was indicating normal and then about 2 miles from the MAP it started fluctuating up and down at half scale in either direction. Now I have been known to have bad days but not a half scale in both directions.
It turned out that an aicraft had joined a tight base leg and flew accross the GS. Had I not been monitoring the DME/Height profile it might have ended in tears!