..This may sound a bit silly, but try tapping the starter with a hammer the next time it doesnt start. It worked well for me (For about a week)
I notced that with my ignition problem, as with yours, it was intermittent and seemed most prominent once the car warmed up
My guess is, in my case, something inside the starter (I.E. an electrical connection or the starter solonoid) was being affected by the heat expanding/contracting wires touching or the soloniod causing a stuck starter soloniod.
I have snce replaced my starter and have yet to have a problem starting the car
(Keeping it on the road, on the otherhand
)