Here is a link to free downloads of the Mercruiser factory manuals. Find your year and engine and it will show you what the part Maxum refers to looks like and how to change it.
http://forums.iboats.com/showthread.php?t=342393
To do that service be prepared to use a pair of screwdrivers to 'leverage" the rotor off the distributor shaft. Mercruiser says that's how you take it off. It's done that way because the rotor is stuck to the distributor shaft with Locktite.
You can inspect the ignition module without removing any parts though, I'd say do that first. All you do is lift the distributor cap far enough to see inside the distributor. You can do that with the plug wires still attached if my 5.7 with Thunderbolt ignition is any guide.
I believe Mercruiser says to check for a good solid connection from the sensor to the board. The connection can fail and split, in which case you can get intermittent problems like Max had.
I found corrosion on the underside contacts of my distributor cap this spring. Check that too. I agree with doing the cap & rotor as a preventive step. They cost about $40 for the set.
I'm with Maxum, fix it as soon as you even think it's a problem, that way you don't need a tow home or worse.
Pete