Out of the blue, I have a track that has some "pad type of voice" that persists in playing.
i reset the track to the bank and voice I want, then it phantom voice shows up again.
 
Any clues?

Probably a MIDI note-on that's not seeing its note-off. As a test, temporarily change all durations to 95% of their existing value and see if it persists. It could be that the note-off happens in the middle of other MIDI events and is missed.

I'm not clear on what your problem is so I will give you an answer to 2 I think you might be saying is happening.
 
1- You change a patch and it changes back again-
This can happen with a down loaded midi file if there is a patch change after the 1st measure. Sadly Sonar does not show PG in the event list. This can also happen if your using the VST to set the patch and there is already a patch chosen in the inspector. 
The settings in the Track inspector are what rule the VST. If there is a patch and bank set that will over ride any changes you make in the VST GUI patch settings. If you wish to choose patches in the GUI set the track inspector to NONE> 
PG changes sometime will show in the track pane view in the middle of a song so look for them. But I have some that are not visable at all anywhere. 
Sometimes re assigning that track to a different channel works. 
Sometimes slip editing the track to the start of the first note works.
 
2- you hear a sound but can't find were it's coming from- It might be a hidden track. Use the track manager to find it.