I'm seeing the exact same thing with my S2.  I have S1s and S3s cranking away at Nicehash with no issues, but my S2s get very high reject rates and huge swings in hashrate.  I'm running the latest firmware and I did try running the stratum-proxy solution Nicehash recommends.  I've played with difficulty and everything else I can think of but can't seem to get stable performance from my S2s at either Nice or Westhash.