Heat Failures

Hey, all: I'm hoping for some info about older micros and heat failure. We have an archaic system at work which is going stupid on us, and we're looking at repair-or-replace options. Let's say you have a system which is locking up due to heat problems. I suspect we've all seen that from time to time. We're talking failures where you can cool things down and reboot and the system comes back and works again, not failures where things are visibly melting and/or burning. :) Is there a "typical" mechanism by which heat causes a system to go south? If you get a system which is knocked out by heat, does it make it more prone to being knocked out by heat in the future? In other words, do heat failures make a system physically more susceptible to more such failures - is the damage cumulative? Any relevant info appreciated. Thanks! -O.-