Thanks for that great idea cajuncook. I'm actually pretty sure now that it is a clutch dog. What I failed to mention in my previous description of the problem is that there is also a loud "clunk" sound associated with when this happens. In fact, the fist time it happened, I thought I had hit a submerged tree and sheared the pin.
I just got off the phone with the nearest boat repair guy...about 150 miles from me...and I described the problem to him. He says it is almost certainly the clutch dog and that I'm probably looking at up to $600. for the repair bill with parts and labor. Not very good news since I might be able to find another used motor for not much more than that. He says he hasn't looked up the price, but wouldn't be surprised if the clutch dog/forward gear assembly will be about $400, just for parts.
According to this guy, if it were a spun hub, I wouldn't be getting the "clunk" sound everytime it happens, so I'm pretty sure now that I'm dealing with a clutch dog issue.
Another idea would be to purchase a remanufactured lower unit. I've seen some available for about $400 or maybe I could stumble across an even better deal somewhere. The mechanic I talked to said that installing a remanufactured unit would be something I could easily do myself without having to bring it in to him. Is this something that simply bolts on and is good to go? No special knowledge or tools required?
I guess I'll need to do a little research and see what I can find. I still intend to get a new prop regardless of what I do about the clutch dog.