1986 OMC 3.0 starts and runs great for 30 min then runs rough, and dies

[Muley hunter 33]

I have a 1986 OMC 3.0 that will start from cold in any given day and runs good for about 30 minutes before starting to run rough and eventually dies. When the engine starts to run rough I will give it some rpm's and it still missed and then when I let off the throttle it dies. It does have the pertronix 2 ignition with the flame thrower coup. What I have noticed is that when not is running I put the red lead of my multimeter to the positive side of the coil and the black lead to a good ground on the engine the voltage fluctuates grately. It bounces rapidly from 13+ volts down to 2+ volts and all over in between.
Now when I connect the red lead to the positive side of the coil and the black lead to the negative battery terminal it does not fluctuate at all. Am I going in the right direction or does this not have anything to do with it at all.
Any thoughts would be appreciated. Thanks

 

[Silvertip]

Are you losing spark when it quits? If so, it's an ignition problem. What are engine temp and oil pressure readings when this happens?

 

[Muley hunter 33]

Not sure if I am loosing spark when this happens, I will have to have someone out there to try and start so I can watch my spark tester. As for the temp, it is at about 170f where it seems to operate normally, and oil pressure is at 35-40 where it seems to stay at normally

 

[Muley hunter 33]

However the oil pressure gauge is very active and Boyce's around constantly while running. I never smoothed out

 

[Yourused]

Dude.. mines doing the same. 1989 3.0 smells rich when sputtering. Rebuilt carb, replaced plugs and wires, coil. Saturday night i noticed when i hit the trim button it died. I checked and sure enough the alternator was bad. So i thought i had finally fixed it. I WAS WRONG. Still think its an ignition issue. It has electronic ignition now. Boat was under water last year before I rescued it. If I can work this out then i got a decent little boat at a great deal.