I had this problem with a futaba 2.4ghz radio system and a orion vortex esc and motor. The solution after the whole time was to just go ahead and buy the little Spektrum $4.99 voltage protector cap. You plug it into your receiver and wolahh fixed. I know you won't believe it's that easy but it took me over 6 months of losing signal the way you are and going back and fourth to a hobby shop untill i found one of these voltage protectors. When i used it I've never ever encountered another problem. Worked for me and im sure your problem sounds the same as mine so it should work for yours.

I hope this has helped you to be able to fix your cars but it's not much to ensure they run everytime