To be fair, they only flicker in the OT. Also, I wrote a huge rundown on the flicker/non-flicker issue a while back. The short version(which could be canon too, really) is; Saber crystals decay. When fresh, they produce a stable saber, but over time, they begin to flicker. Case in point: Anakin's saber in ROTS is pure blue, and doesn't flicker. In ANH, when Luke receives it, its color has faded, and it's flickering. Kinda makes sense.