It is a myth. there is no evidence for it. there is no medical plausability, shall i go on......

A myth. Physically impossible. An excuse used by some incompetent and/or compromised investigators. Name one person who died from heat stroke who also spontaneously burst into flames. However, it does make for good TV ratings.

Well, there is this:

[trace.tennessee.edu]

People don't seem to believe in it the way they used to, maybe because of the lack of evidence. From what I have read it seems difficult to find a genuine case where an external source couldn't have caused the fire. Perhaps the current increase in willingness to dismiss evidence will make it more likely to make a come-back!

I think it would have to be a volatile mix of chemicals either ingested or produced naturally, or a combination of both that can be the only cause. But it certainly, if it's not a hoax of some sort to cover up suicides or murders, has a scientific explanation, if it's real. Doesn't seem to be very common though, and the evidence is probably destroyed in the process. Also, modern forensics sciences would be much better equipped to deduce the source of combustion nowadays, which I suspect is also the reason for the current lack of viable evidence to support the theory.