It's simple. The OVP protects some expensive control modules (engine management, ABS, traction control, etc.) from being damaged by high voltage conditions. A typical over-voltage situation could be caused by a faulty voltage regulator, jump-starting, or reverse polarity. Without OVP, the repair bill would be astronomical. In your case, it sounds like the OVP is intermittently cutting power, or at least lowering the voltage to the engine control module, resulting in the crank/no-start condition. This is a very common problem. Follow Arthur Dalton's advice.