Cold Start Trouble

98 Dodge Grand Caravan SE 3.3L V6. 98K Engine won't start. Battery, alternator, and starter tested good by a local shop. Symptoms varies. One day, when key is turned forward to on position, I hear a hum; other days, a click, or a hum and click, slight crank followed by a click, sometimes no sound at all. But the engine eventually turns over and run after several tries especially after turning key backward serveral times before turning forward, then it is good for the day. Any ideas, Thanks in Advance.