I normally change wires when I change the cap & I use a cap that has the brass posts instead of the general cap that comes with aluminum.
Besides the cap, here are other things to consider:
2. Rotor - also should be changed with cap,
3. Points & Condensor (Capacitor) if you have them should also be changed at the same time,
4. Plugs - could be fouled, cracked, etc. Check their condition and at least clean & gap them to spec,
5. Coil - usually this either works or not.
Not knowing what year it is and the motor size it's hard to be more specific, but you could also be having carb problems or sensor problems if it's a fuel injected model. Some sensors that cause poor starting & running include:
1. Throttle Position Sensor,
2. Idle Control,
3. Crank Position Sensor.
Also, check all vacuum lines to ensure you have no gaps/breaks in any hoses and all unused lines are plugged.