1998 Nissan Maxima Intermittent Failure to Start

My daughter's Maxima has an intermittent problem. So far we have replaced the battery, starter, and the sensor that interlocks the automatic transmission shift (to avoid starting in any postion except park). There seems to be no pattern to the problem which may go for a month or two without showing up. Two different mechanics have tried to debug the system but the car will not fail to start when they have it in for check out. The car's electrical system is
up and running (stereo, lights, etc) when the failure occurs. Most of the time the car has been started and run some time before the failure so it does not happen on a cold start up.
Help!