I am resurrecting an old thread here, but just to ask whether you mount the esc directly on the chassis or anything similar? I know it sounds stupid, but it seems that the way you do it puts too much stress on the case. Have you tried to mount it on a plate which sits on top of foam pad or similar? maybe you are just tie the zips too much and negate the foam flexibility in that way.
Another stupid thought is that some wires or something is constantly pulling the esc in some way under running conditions. It looks like lack of flexibility to me.