I recently saw a similar thing happen to someone at my club. However they where lucky and 2 days later someone who had found the plane showed up at the field with the intact fuse and the wing broken in half (It had jammed into a tree). They inspected the cabling and found out the connection between the battery and the RX was faulty and had been the culprit of the misshap. Perhaps the vibration can eventually unfasten the battery from the RX causing a loss of control. If the plane flew straight, as it was the last time it responded to an input from the TX it sounds as the RX in fact loss power because electrical lines and other factors that may cause interference will in most cases cause the servos to act in random ways which will be seen as erratic maneuvers rather than smooth straight flight. I'm sorry for your loss.