i have had the very same problem, had three runaways during some practice and one after a qual run. i have a zo1b with a 603fs rx and it also happend in columbus a couple weeks earlier while scott cramer who is with futaba was there and he said heat should be an issue??? so what i did was take a heat gun to the receiver and at 150-160f the light went from green to red everytime. i dont know what the failsafe mode is supposed to do but mine ran away as i was holding the brake to go pick it up off of pit lane, i mean went to wot and was gone!!!
i was told that this problem was cured with the 604 rxs and someone loaned me one but we could not get it to bind with my 3pk. so i drilled the holes as mentioned above and put the 603 back in and ran a 6min qual and 20 min main with no issues. i would rather not have the holes in the rx box, but i cant deal with the runaways so guess that how its got to be, but that amount of heat should be tolerated by the rx in my opinion, im using a nimh batt for power. just thought i would share what i found.