i had these same issues after doing some hard bashing. my conclusion was the shock of the landing was causing the esc cases to flex into the boards causing a break in some of the internals. i went through 3-4 escs that way.
my solution was to make an esc mount that was fixed to the chassis that also had a ledge that didn't touch it either. that way, if i had a hard landing, it could flex several inches and never touch anything that was taking the shock of the landing.
you can make one for about 5 dollars out of anything that is rigid. i used aluminum.
here's my quick drawing of it....it seems to work extremely well as there have been no more esc issues at all.
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