Updated to add that this document has now been released as a Technote. A couple of alterations have also been made.
I hope the attached document may be of interest. It is an attempt to highlight the differnt areas that may be considerend when investigatting an issue.
There is not really any right/ wrong answers, so I'm not currently looking at making any major changes. The idea is more of a demonstration of how you 'might' tackle an issue, it is up to the individual to 'fine-tune' the exact methods they use.