Looking good. How's this? I'm not sure about adding some restriction to their maneuverability.
Movement Actions Only (Ex): Dashers must move at least their speed in any round, and they may only take double move, charge, or run actions.
Meltdown (Ex): Any round that a dasher does not move at least its base speed, the dasher loses one-third of its normal full hit points each round. If it reaches 0 hp, it is destroyed completely and reduced to molten slag due to the extra heat generated by its power source.