2001 Buick starting problem

I have a 2001 Buick LeSabre Custom, 6 cylinder. The car will not start if it has less than a 1/2 tank of gas and it has rained. It will start if only one of these conditions exist, but not both. Otherwise the car runs great. Can anyone please help?

Have you check fuel pressure when it will not start? Do you here the fuel pump prime for 2-sec when you turn the ignition on? Is the drivers side carpet wet?