[Milhouse]

Stop overclocking sdram_freq - it gives practically no benefit, but can often result in instability with recent firmware.

I too had a completely stable Pi2 for months with sdram_freq=500, but recent firmware resulted in random crashes and freezes, which went away once I stopped overclocking sdram_freq.

You could possibly regain stability by bumping up the sdram voltage, but unlike the Pi1, the Pi2 benefits very little from overclocking sdram (mainly because the ARMv7 SoC has increased L2 cache) so ultimately the increased risk of instability is just not worth the hassle for the meagre improved performance (which you probably won't even notice).