This is something I wrote in response to another question about tummy troubles and running - I thought I'd cope and paste it incase there's something of use for you! 
When you undertake sustained activity - typically 60 minutes plus, several things happen that can affect the way your gut works.
Firstly, blood flow to the gut is reduced in favour of increasing blood flow to the working muscles. This can cause GI symptoms including cramps, nausea, vomiting and diarrhoea. Secondly, you get dehydrated. Dehydration further reduces the flow of blood to the gut which can exacerbate GI symptoms. Thirdly, you get hot (even in British summer conditions!), and this - guess what! Further reduces blood flow to the gut 
So lots of things going on in the way the gut is working.
When you consider putting something in to be digested - there's obvious scope for even more problems. However - and here's a key thing to remember - one of the best ways to increase blood flow to the gut and to reduce some of the problems of dehydration, is to keep stomach volume relatively high. The more there is in the stomach (up to a point - you don't want to be gorging yourself!) the greater the flow of blood. The easiest way to keep stomach volume high and to reduce the effects of heat stress and dehydration is to drink. Water with added electrolytes as a minimum, but adding some carbs in will obviously help to keep energy levels up (either a sports drink or gels/shot bloks/similar along with some water). If you do want to take on carbs - recommended for marathons - then you need to practise and train the gut to tolerate whatever it is you do.
Going back to the fibre issue - when you combine a fibrous diet (which gives the bowel a lot of work to do) with activity that reduces the efficiency of the bowel, then your almost setting yourself up for a problem. This won't apply to everyone - some people seem to be immune to tummy problems - but as you have a history of IBS and ongoing problems with runner's tum symptoms then it would be number one on my list of contributing factors.
I'm not advocating cutting the fibre out of your diet - as I said, it seems a very healthy diet. But you may want to look at the timing of what you're eating. Avoiding certain foods in the hours before training and the day before a race may well help. It's certainly worth experimenting with. Try these links for more info.
Runners gut
Low FODMAP
Another thing to consider is the mechanical 'jarring' that comes from running. Some people seem to be more susceptible to this than others, and there's a school of thought that proposes having a strong core can 'buffer' the intestines. So lots of planks and the like may help.
And finally - caffeine can easily irritate the gut. People with IBS and other GI complaints are often advised to cut caffeine from their diets. As you don't have tummy troubles outside of running it probably isn't necessary to cut it out completely, but avoiding it the day before/day of a race may help, as may avoiding it in the hours before a training run. It may be something worth experimenting with.
