Latency's most likely the issue you're getting, or an unreliable internet connection (I'm guessing it's the former). Ethermine seems to be having stale share issues anyways (perhaps their Stratum servers are overloaded?). I'd personally try moving to a different pool temporarily to make sure the issue isn't on your end, though I'm pretty positive the issue lies in Ethermine rather than your connection.
thankyou for the reply,
before one month i got around 2-3% stale shares.from the middle of February i got this problem.it also reducing my valid shares.