I have a mercury grand marquis 1996 wont start but then it does on his own

I had been taking the car to the mechanic with the same syntoms; it cranks very well
but wount start. He replace the oxigen sensor becuase check engine light was off and
it was detected that sensor was malfunction. The car started doing the same thing but
few days later started again on his own and the sensor light stays light up again.
Before the last time this happened the car was taking to the mechanic via towing truck
as soon as we got there, the car started like nothing ever happened. I reach the
conclusion that the fact the car was moved while in towing truck, the problem is the
fuel pump. The mechanic discard the fuel pump as been the problem instead replace
the oxigen sensor. Two days ago the car started doing the same and today started like
nothing ever happened.  Dirving me crazy!   Any idea?