Thats only happened to me if my HW is high. Did you check that? Also, which middlecoin are you hitting? Maybe the one you are hitting is getting a bad ping or being worked on. Have you set up a failover?
Having a strange issue -- wondering if this happening to anyone else: I have a 2.5 mH/s rig and for some reason middlecoin is reporting 0.5 Mh/s. I've been on the pool for weeks and today its showing less than a quarter of my hashrate.