Hi I have IBS(d) too. It is usually the fat content in crisps which causes upset. Food I also have to avoid if I don't want to be stuck to the loo for a few hours the next morning are celery, beetroot, bananas (surprisingly!) peas, beans and potato in too large quantities. I also have a sensitivity to soya in all forms but can cope with a small amount of bread each day.
Taz x