having problems w/ my '91 Maxima

wats up people,,,im having some problems w/ my Max. Ive been having this problem for awhile and i was told it was fixed but its not. Every once in awhile my car decides it doesnt want to start. It will turn over just wont start,,the battery is not dead and there is gas in the car. I just changed the oil, spark plugs, spark-p wires, gas filter. i really dont know wat to do. plz any help would be appriciated.