Its related to the malleability attack[1]. Newer version no longer relay modified transactions to lower the impact of the attack. Once all nodes are updated it will be impossible.
That was helpful too. Thanks, shorena.
I'm observing such a behavior for the first time. For instance as we speak, for 20 minutes there wasn't a single block synced. After that peer list was reset entirely and slowly populated again. Took another 5 minutes during which time 4 peers were disconnected. Took another 10 minutes to have just 5 more blocks synced from the new peers before they stopped responding. Now awaiting the peer list reset again... At such speed blockchain syncing can take more than a week. Definitely there is something wrong.