Intermittent faults like this can be difficult to pin down. The only time I've seen something like this was when I was editing in Avid Symphony. One of my linked/AMA'd clips would occasionally not display and cause the software to hang for around 20 seconds, after which I'd get an error message.
It turned out that there was a corrupt frame in the middle of the clip. If you loaded the clip from the start, all was fine. Once I'd played across the corrupt frame, my problems started.