Users of several forknote-based coins have noticed their daemons getting stuck for several hours while trying to keep blockchain synchronized. Throrough investigation have lead the main culprit being a flaw in handling of Zawy difficulty in alternate chains.
Root cause of the inability to sync the blockchain is some rogue mining pools creating large number of blocks without Zawy difficulty algorithm and daemons using Zawy difficulty algorithm trying to keep up with both chain branches but failing due to numerous chain splittings causing daemon running out of physical memory, and exhausting disk I/O bandwidth due to simultaneous swapping and database access.
Workaround for the issue is stopping the daemon and deleting the database, adding checkpoints in the config file for blocks right after the split points from official block explorer and restarting the daemon. Daemon will recreate the database from blockchain storage using only blocks from current chain. This will work until there is long enough alternative chain again.