1998 Dodge Ram 2wheel Drive 1500 5 2 First Would Hesitate Stall Most Time

I have a 2wheel drive Ram 1500 with a 5.2;At first it would hesitate and stall most of the time when trying to take off and was also very hard to start.Now it wont even start.I put new plugs,rotor,and cap.wires tested ok.When testing for spark at each plug,what would cause the strength of spark to vary?ex. weak weak strong weak