I have a sporadic starting problem

Over the past 8 weeks I have had 4 times when the engine turned over but wont fire. I have had 2 mechanics tell me that it is the fuel pump. One mechanic says that the armature on the fuel pump motor has a dead spot and that is usually the problem, if that is so, then why does it start right up later. Sometimes as quickly as 15 minutes, he then told me that temperature fluctuations can cause the problem. I don't want to spend $400.00 for a fuel pump assembly for a "might be problem". I've changed plugs (new that wasn't the problem, but did it anyway). I also have added a "G-Force" chip to the map or Iat sensor, but unplugged it after the first no start problem. The only other thing I have changed is that I installed a "K & N" air filter. I paid the online mechanic $18.00 for his answer "dead spot on armature".