Shower used to work good. Now a small turn of the hot knob makes the water either too hot or too cold. When the temperature is set right, the hot water tap is barely open. The shower is probably about 40 years old. What would be the problem with this?

Have you considered replacing the shower's hot water vale, maybe all of the valves.

There is probably a ton of crud in the line. Also make very sure you get the exact model of valve for replacement. I suspect that the real problem is the obstructed flow of cold water. Small changes in the hot water valve might be caused by the mixing station (I think that's what its called) not getting enough cold water.

Both hot and cold have close to the same pressure when going through the lower spout. The pressure for both hot and cold is about the same when going through the shower head as well. That is if I turn either the hot or cold off completely and turn on only one at a time. I have no problem mixing hot and cold when I want the water to come out the lower spout. The mixing doesn't work well only when I want the water to come out the shower spout. When I turn on the shower spout, all the water comes out the water spout (none out of the lower spout). Is it possible to repair this without having to pull apart the wall? Is the problem with the diverter? Do you still think it could be clogged?