I was suffering continual lockups and having to force quit when attempting to link to a particular file. This file linked OK on two other systems. Then other links failed and I was having to force quit, time after time. In the end I deleted HD>Users>Shared>AvidMediaComposer>AvidSourceBrowserCache. I also deleted AvidImageCache for good measure. All is back to normal. I suspect that this problem may arise because I have to link to the same files, time after time, in different classes, when teaching, but I am not sure of that.
MC 8.6.4 and 8.6.3, MacBook Pro. OS 10.11.6.
Hope this info helps someone.