OK, I am going to go on the assumption that it is a bad voltage regulator. I am going ot give you a series of tests, and what the normal results of those tests should be. At any time you get a result significantly different than what it should be, stop and let me know. We are actually going to test all components to make sure they are good. I do not like changing parts unless I know they are bad.
First, remove the brush cover. disconnect the wires going to the brush leads and ohm across the brushes. You should get 6-7 ohms.
Place one meter lead to one of the brush leads, and the other meter lead to the frame of the set. You should get no continuity.
Reconnect the wires and set your meter to VDC. Place one probe of the meter to each brush lead and start the set. You should get 12-20VDC. Stop the set. Disconnect the wires again.
Remove the control panel and locate the voltage regulator at the back of the control box. It will have a 6 wire connector with the wires being red, white, brown, light green, and 2 yellows. Unplug this connector.
If you have electric start, you can use the battery on the set. If you have a manual start, you will need a 12V battery. Make up 2 jumper wires about 3ft long, and attach a connector the same size as the spades on the brush leads to one end of each wire. The brown wire is the positive brush, and the light green wire is the negative brush. Attach one of the jumpers to the positive brush and the positive post of the battery. Attach the other wire to the negative brush. Start the set and attach the wire from the negative brush to the negative terminal of the battery.
Go back to the 6 pin plug that you just removed from the voltage regulator. Set your meter to VAC and connect the leads across the following pairs of wires:
BE CAREFUL. THERE WILL BE LIVE VOLTAGE PRESENT AT THE PLUG.
Red to white should be 110-126V.
White to light green should be 122-142V.
If these all check OK, your voltage regulator is bad.
Please let me know what you find.