Intermittent won't crank
 

Help! I have a 1999 F-250 light duty 4 WD SC Lariat with 5.4L motor and AT. Usually it works fine but every 20-25 times I try to start it I get no cranking. The guages and lights all work but the radio doesn't work when this happens. I can get under the truck with the key in the on position and jump the contacts on the starter solenoid and the truck will start and run. The battery has been tested and is in good condition. I tried checking fuses and shaking the wiring harness but have found nothing. Any ideas? Thanks in advance for any help.

Your silenoid should be under the hood either on the firewall or on the inner fender. Not under the truck except on Chevrolet and I think Dodge too is mounted on the starter. ;)
If you can jump the starter silenoid and it starts then it is the silenoid. Wifes Explorer has a chronic problem with it once or twice a year. $18 and 5 mins to change.