When it doesn't start, does the car attempt to run at all some of the time?
I had a similar problem but my car would run reallllly rough right before it quit. Replaced the crank sensor - it worked for two weeks then did the run rough and stall intermittently. Replaced the PCM/ECM - worked for two weeks then ran rough and quit. Replaced the crank sensor again - worked for a day. Replaced the PCM a SECOND time and, knock on wood has worked to this day.
It was a bad crank senor that somehow ruined the first ECM, and the second ECM was a dud. Not to rain on your day, but watch out for those reman-pcms.
'94 Bonneville SE - 106k miles 6/1/07