Mining stuck at new block

[Zarmung]

Hello.

Will decribe the situation which regularly occurs: I mine an altcoin, all is fine but sometimes happening strange - mining stuck(miner works but couldn't find or calculate a block). At times it takes 10 hours to go forward and all this stucking time the client(wallet) couldn't Sync with network(but i see alot of active connections) then after some time some1 finally finds a block, miner detecting new block, wallet direct Synchronize with network and all is fine.

Any idea why it happens?