Usually a runaway is caused by improper trigger manipulation which leads to worn parts.
I've had them runaway when the barrel got red hot. I remember one night when the belts were linked and the barrel got so hot that you could see it glow across the LZ. At that time in my career, I was playing with the foot-sloggers and never really liked the M60. Later I transitioned into tanks and when I got my own track, I really liked the M2, but I never had much use for the M60. (In full disclosure, when I was in the infantry, I wasn't a maintainer, simply a user. I couldn't tell you why a M60 sucked, I just knew that it did. When I got into tanks, I learned the value of maintenance and tried hard to learn all the ins-and-outs of keeping the vehicle moving.)