It's hard to diagnose such a strange problem, which is probably why I couldn't even think of the answer until now. I have had the same symptoms as you and fixed it with an ignition switch cleaning. Not sure how I haven't remembered this issue after all the trouble it caused me the first time, but it sounds just like what happened to me. At first it was intermittent, key on- crank but no start, key off, key on- crank but no start, key off, key on- starts right up.
Try hitting the brakes while you have the no start condition with the key still in the on position, if the dash lights dim (more than normal), the ignition switch is positively dirty.
If they don't dim, I'd clean out the switch anyways, easy insurance. ~30 minutes.
http://www.eilenberger.net/K75S/IgnitionSwitch/index.html