Along with bad RAM and ROM chips, IC sockets making poor contact is definitely at the top of the list of common PET problems, especially if they've been disturbed; I prefer to slide the chips sideways rather than remove and replace them, if only to avoid the possibility of bent or broken pins. But in my experience with dozens of PETs and AIM65s, some working in extrememely hostile environments, I found that a remarkable product called Stabilant-22 almost totally eliminated contact problems (unless there was actual damage), and virtually eliminated frequent socket-related service calls. I don't recall having to replace a single socket; the only issue with certain socket types is that with a lot of vibration or circuit board flexing the chips sometimes migrate out of their socket. It's expensive but it does work and a little goes a long way.