Main relay, fuel pump or what?

[solanog]

It has happened to me about four time in the last week. When I try to start the car it won't start. I hear the clicks of the main relay but can't hear the fuel pump while the check engine light is on (couple of seconds after turning the ignition on). I turn the key the car starts for about half a second and dies. Then I keep on trying, even hit the tank and it starts only when it wants to. Once it starts I have had no trouble (and hope it stays like this at least).
This does not happens always, I'm not sure but seems like is more common when little fuel is left on the tank, so how can I determine what's the problem?
The car is a 1999 Civic. BTW a friend told me to tear of the main relay and re solder all the contacts, I did it but I had the failure once after that.