I just went through this problem with a new guy at our field. I test flew an Eflite L4 for him and lost link about 100 feet away.
This past Saturday he brought the repaired plane back to the field. I insisted on another, more rigorous, range test this time. While I was walking away I lost link a couple times. I finally tracked the problem to the battery box. The batteries weren't seating firmly and the Tx kept rebooting. I removed the lower metal battery tab and put some small bends in it to help make better contact. That helped for now but this is a mechanical problem that is causing crashes. A minor redesign of the battery tabs would fix this.