Gene,
I figured out the problem and the refrigerator is working fine. You can put this one in the boneheaded move category.

I use this refrigerator to keep kegs cold, so I have a piece of plywood in the bottom to make a level surface. I have had the plywood in there for several years without incident. After thinking about the return air path, I realized it must be under the plywood. So, I removed the plywood last night, and this morning the refrigerator is down to a frosty 35F.

I'm not sure what happened. I must have shifted the plywood just right so it blocked the return vents.

Anyway, thanks for the help. On the plus side, I now know how the temperature control system works in my refrigerator, so I should be able to figure it out with less effort in the future.

thanks again,
Dave