[macshpman]

If the vent is clear I would lean towards electrical. Coil could be the problem. I would lean more towards that then the cap, rotor, wires or sparkplugs. They would tend to act up all the time and not just now and then. I myself would change the pick up in the distributor first. Dont let the mechanic tell you that they are either good or bad. I personal have seen them work fine untill they get hot and then stop working. Once cold again it will work. I believe mercruiser has an update pickup because of this problem. Good luck