My '94 Ram Van B250 always cranks but won't always start

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When the check engine light comes on it will start but the light doesn't always come on. The engine will also sometimes die while driving or idling for no apparent reason. The van may start again in 10 minutes or it may take ten days. Not a fuel or battery issue. Have replaced distributor cap, plug wires & crank sensor but having the same problem. Coil is good also, but it only has spark when the check engine light is on. There doesn't seem to be any rhyme or reason to when the light comes on and when it doesn't. Have checked all fuses and cleaned the control module connections and the battery ground wire connections. Checked the codes but not sure I got them all correct. Possible codes are Solenoid Coil Circuit, Throttle Position Sensor, ASD relay circuit, Fuel Pump Relay control circuit. Does this model have a Z1 voltage I can check? Any way to pin-point what the issue is? Thanks in advance for your help!