1994 Ford Taurus Starter relay wont engage unless engine is

My problem on my 94 taurus GL 3.0 liter is that intermitently when i turn the key to start it i will not hear the fuel pump engage and i will hear only a small click from the starter solenoid. I then hop out of my vehicle, pop the hood and manually turn the engine over by pulling on the serpentine belt. This occurrence happens about 1 in 20 times. The fuel pump will then presuure up .Then the car will start and run perfectly. What causes this problem ? I have clean and new electrical leads with good contacts. Baffled in Alberta Canada .....

George