2004 Mercury Gand Marquis will not start

Searched for: My neighbor has a 2004 Mercury Marquis that has and intermittant starting problem. It will operate fine for 3 or 4 weeks, then you go to start it and it will not start. It turns over fine. Battery is good. I tried battery disconnect cycle thinking it might reset the computer logic, key on and off several times, and checked fuses. One time it started right up after that. The next time no help. It was towed to a mechanic and when he got in it, it started. He did a diagnostic on it and determined it was a crank position sensor issue and replaced it. But, 3 weeks later it  happend again. I have heard that some makes of cars need to clear old timming circuit faults in the computer to function properly. Is there a manual way that I can try that with this car or do you know anything elase I can try. I think there is a maintenance bulletin out for starting problems on this car, but no recall to fix it.  Note:  I got it started this last time by removing the cam sensor and crank sensor and reinstalling it along with a battery disconnect reset.  it worked, but 3 days later, it would not start again. Now that doesn't work either.  Thanks!