Another thing is to properly set your failsafe mode. On my spektrum stuff, you have to hold the brake when binding the rx to the tx. That will cause the car to hit the brakes when it loses signal.

However, Im willing to bet you have a loose connection the battery. That is usually the cause for a runaway with 2.4ghz. I've seen it many times. The connection may seem snug but it may have dirt in there or the wire has a loose connection elsewhere