I have a '88 FLHTC. I replaced the stator, rotor and regulator a month ago. On a ride a few days ago I noticed that the voltage gauge was around 10v and wouldn't go up as the RPM's went up. Tonight I went to start it and it didn't have the juice to start. I charged the battery and it fired right up. The battery is only 3 months old by the way. So I started to look for what might cause the problem. I found that the single wire from the regulator to the battery had rubbed through the insulation on part of the frame. I hooked up the old regulator, which was still working when I replaced it, but nothing changed. Did the wire rubbing on the frame cause the stator to go bad?

 

Make sure the regulator has a good ground - no paint blocking the connection.

If you're worried about the stator test between the stator leads for resistance and check to spec. - I think it's .1 - .2 ohms. Then test each lead to ground for continuity - they should be open.
then set meter to AC and 2000 rpm the output should be in the 32-40 ac volts (if I remember correctly)

 

It was been correctly suggested that this could be possibly a ground issue. Since ground issues generally make you feel like you're having spikes in your voltage reading and the reading is not as stable, you can easily judge it from that. However, you can simply check where your wire is grounded and whether there is a short or not.

The only other reasons could be a bad battery or a bad stator. Since a bad battery is not the case and it wouldn't be able to start your bike if it was actually at 10V, kindly check your wiring.