Could you check in the logs whether there are any "Block fails to get added" messages? It's probably easier to spot if you restart monerod with --log-level 1.
I've tried --log-level 3 as well, but it's the same deluge.
However, with --log-level 2 it's easier to spot the important things. What intrigues me is this message:
2017-05-16 20:44:22.197 [P2P8] INFO net.cn src/cryptonote_protocol/cryptonote_protocol_handler.inl:981 [IP:50350 INC] Block verification failed, dropping connection
"IP" is different every time. (over a couple of minutes worth of logs this is present dozens of times)
I don't see any messages like "Block fails to get added".
EDIT: Actually, this is even more relevant:
2017-05-16 20:41:19.797 [P2P9] ERROR verify src/cryptonote_core/blockchain.cpp:3185 Block with id: <82e8a00ebb396dcafc9ba5b44e9aad367bbc8f886736b0a49dd506c114b621fa>
does not have enough proof of work: <2e55a37fb07a0813b6fa36e541e677d538c9d6b8105f6d60e07682af4239abfb>
unexpected difficulty: 60
And this repeats for every peer.