A couple things to check:
-You may want to try calibrating with you EPA's set to max
-It sounds to me like your LVC (low voltage cutoff) is kicking in. The LVC is there to protect your battery, as LiPo's cannot be discharged too far or they will be damaged. I don't know what the settings are like on the Castle, but you can usually set ESC's to auto-detect your LiPo cell count or you set it manually. The ESC needs to know how many cells the battery has to accurately apply the LVC. For example, 3.2V per cell is a common cutoff, so if you are running a 2S pack your lVC would be set to 6.4V. If it was set for, say, a 3S pack then it would want to see 9.4V or more to function. If you are running a fully charged 2S pack you will have 8.4V, so if you were in 3S mode then it would cut out immediately.
I would re-check your calibration but what you say about the motor starting for a second and then stopping to me sounds like an issue with your lVC. Do you have a program card or USB link for the ESC to check the settings?