PSC that makes sense. I've suffered from a stitch running the last 6 months or so but I'd put on about a stone in weight. I've now lost that again and the stitch has stopped happening which I suspect ties back to what you are saying.
It happened at Richmond on Saturday but that's the first time its happened to me. I think the whole race was out by about 1 minute. Anyway its fixed now and I'd say, so what? as long as the right answer eventually prevails.
the 5k suggestion is a great one.. I was a 2:00 - 2:10 half marathoner but was only training distance. I started running the free Saturday Parkrun 5k at Richmond and, because it is short and you'r running against others, you naturally run faster and 5k time improves.
A few months later I did the Half in 1:48 at Henley and 1:46 in Amsterdam a week later. It works.