Simple fix is more power, forget batteries get wall plug in DC adapter 3VDC@500mA run wires in safe place. Fixed supply current will extend motor life and control motor speed. If you must use batteries go with a lithium type battery that is rechargeable when light flickers or dims recharge. This time should be greatly increased with lithium battery only. Purchase new battery and charger use correct charger together only.
Last never give in never surrender, never forget, Ohms Law double the current any type battery simply parallel two three more batteries , you get more power, you must have more power or current in E= I *R.