I replaced the controller board. The voltage fluctuations are now about 0.5 volts DC, and the saw runs noticeably smoother. However, after running for about an hour, it stopped while running at full speed.
Voltages are good to the brushes. The commutator looks good from what I can see through the brush hole. I connected an ohmmeter to the motor leads, slowly turned the motor by hand, and got random readings. When applying power to the motor at the slowest speed setting and turning, it would start and run for a short period and then stop. The same thing happened at high speed. I suspect the brushes and have ordered a new set from Seyco. The brushes are not listed on the parts page. You need to call and ask for them. Brushes are cheaper than a new motor.
This photo shows the controller boards with the new one on the left. Note the sealed trim pots on the old board. I had to adjust the new board to get the same voltage as the old one. The motor is a 60-volt motor, but the most I can get out of either controller board is 50 volts. I have yet to find out what the correct output is. The trim pots adjust the maximum and minimum voltage output.