[ford38v8]

Your wiper motor paddle has dried out. You can try lubricating it by spraying oil through the vacuum intake on the motor whil operating the blades by hand. You can fool with the thing in other ways also, but eventually bite the bullet and send the motor out to be reb uilt. Kent Jaquith replaces the old hard paddle with neoprene for a lifetime fix:

Kent Jaquith, 2633 SW Obsidian # 15, Redmond, Oregon (541) 923-4319
Clean-Sweep Windshield Wiper Motor Service, Sales and Rebuilding