What you described is definitely a failing voltage regulator. You can see how they wear if you compare the contacts of the new one to the old one. They're not hard to get at, it's screwed onto the back of the alternator. It might be tricky getting the top screw out, so be patient and work it gently so you don't strip anything. If you can't get it out with a screwdriver, than a set of ignition channel locks will do the trick. Hope this helps!
Precision Motorcars, Cincinnati, Ohio
'67 250SE Cabriolet
'93 300E 2.8