Hey I spoke with another old guy tech. on this problem. He said in addition to what has been posted that he had an alt. that had bad brush spring in it and it caused the same problems.
It made the brush jump up and down and caused radical swings in the amp guage and the headlamps would dim and brighten.
Old guy technitions (excuse me, MECHANICS) are invaluable. If is really weird they have probably seen it.
The differance between a tech. and a mechanic is; the mechanic can listen to it and tell you what is broken, the tech. has to hook it to a machine first.
