Good news!

Starting checking out the system today using everyone's ideas and
traced the problem to a faulty 50 amp circuit breaker, i.e. one of the
terminal studs in the back were loose so contact was poor/
intermittent.

All your advice was very helpful. The idea the finally solved it was
turning on the blower and wiggling all the wires until something made
a difference.

Thanks to everybody for helping with this!

Our next job is to do a full tune-up as it is running a little
unevenly. We replaced the plugs, cleaned the air filter, tightened
the belts and cleaned the rotor and distributor cap. Any other
maintenance work that might make it run more smoothly? We will buying
new rotor and cap when we get to a dealer.

I have a timing light here but have not looked for the marks. What is
the correct timing setting?

Also, the trim tile guage has been totally dead since we got the boat
in the fall. The fuses are ok behind the dash. Where is the pick-up
for this guage on a Volvo Penta drive? What is likely the problem?

Thanks!