If I understand you right, the problem is at certain times the engine will not crank, that is turn over when you turn they ignition key to START. And at times it will start and run normally. Offhand, I would discount the possibility of the Ignition Module. To my knowledge it has nothing to do with cranking the engine but rather providing the spark at the plugs through the coil.
Having said that, might it be that the issue is with the starter system. The starter system is initiated at the ignition switch when the key is turned to start. At this point, the ignition switch then provides +12VDC to a starter relay. The relay is but a remote switch with slightly higher amperage capacity. The starter relay then engages and supplies +12VDC to the staring solenoid. The starting solenoid then performs two (2) functions. It provides the switching action (hundreds amps) from the battery heavy cable to the actual starter motor. The solenoid likewise pushes a small pinion gear along the shaft of the starter motor to engage the ring gear. The ring gear is of course on the edge of the flywheel which in turn is attached to the crankshaft.
Starter system problems could be corrected by:
1. check battery, battery cables and connections;
2. charge the battery if required;
3. clean brushes and guides of brush holders of the starter motor;
4. clean the commutator and commutator segments;
5. check/replace the armature or field coils if determined to be defective;
6. in some extreme cases it could be the drive pinion or the flywheel gear ring is defective.
7. often it is the big switch that the solenoid activates that looses contacts causing no +12V internally to the motor.
8. Most likely would be a defective starter relay or the IGN switch itself. Relative to this, you might try and check the connectors/terminals and electrical joints.
In the picture below, the upper portion is the starter solenoid while the one below it is the actual starter.
Given the amount of work to overhaul a starter, you might consider just replacing it. This page
might be informative.
Hope this be of initial help/idea. Pls post back how things turned up or should you need additional information.
Good luck and kind regards.
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