Erratic spark?
 

I am considering buying a 2008 Mariah SX19- Mercruiser 4.3L. The existing owner says the motor has an erratic spark, and cant turn on. He has already replaced the following items. .Harmonic balancer, Crank Sensor, Distributor cap and rotor. Ignition coil. The ECM box was checked/verified independently as being ok. He is finished with it mentally... He is willing to let me take it to a marine mechanic for a diagnostic. Which of course is the best $150+ bucks you can spend before you buy something. Has anyone had a similar experiences? any helpful guesses? Thanks LW