Check the charge state of your battery pack, and check all the connectors in the path betwixt battery and motor. It sounds like the Electronic Speed Control thinks your battery is low, and is cutting power to the motor. This will preserve enough power for the receiver and servos for a controlled landing, and is called Low Voltage Cutoff.

Low voltage from the battery triggers the LVC in an ESC. If the pack isn't properly charged, or if a poor connection somewhere offers enough electrical resistance, it happens at the beginning of the flight, rather than at the end.

After the load is removed from the battery, it will recover a bit of voltage. In some ESC's the LVC then disengages, and you can throttle back up. Useful for stretching a glide back to the field, and confusing newbies with power issues.

Tell us what charger you're using, and how you're using it. And what battery pack; what you have may be unsuited for your purpose.

Hope this helps,

Steve