Not an electrical engineer, but I was 2 sentences in when I declared to myself "its the external clock source" and then a physical shudder as I remembered how many times a contract manufacturer has used some aging crystal or third party component src and not updated the matching capacitors, causing very similar issues on projects I've worked on in the past. The very very worst is the aging xtals - a warm wind can make them work for an undetermined amount of time, which is bad if the factory is running warm around 26C or so and you end up shipping the batch to Europe where its early winter...