Alternator or Voltage Reg or are both causing this?
 

Yesterday I noticed on my 1982 with a 78 engine and tranny, 911SC, the RPM needle jumped way up to 8000 RPM's then went to zero. At the same time i noticed that my radio cut out. Eventually all systems came back to normal.

I went to this forum and did a little research and used my multimeter on the battery. When everything was operating normal the meter registered 9.6v.

I was fortunate to catch the RPM and radio shut off instance again and I put the meter on the battery again. This time it read 13.7.

Today I was driving and it did it again. I pulled into a parking lot and turned off the engine. Waited 20 seconds and no start. I called AAA, an hour later I tried cranking her and i got a little response but died. When AAA got there we gave it a jump and turned over. I noticed that the RPM gauge was working as was the radio.

So, my question is what needs to be replaced? Alternator, VR or both?

Battery first
 

Pull out the battery and have it tested first.

Do you have an external voltage regulator? If so have that checked.

If you have an internal voltage regulator, pull out the alternator and have them checked.

It will be one of these 3 things and maybe all 3.

A bad battery may damage an alternator. A bad voltage regulator will damage a battery which in turn may damage the alternator.
My guess? Bad battery which cooked your alternator.

Okay. I will take them out and have them checked

What am I checking for and how on the Battery, Voltage Regulator and Alternator?

9.6V is too low. 13.7V is about right but you are saying thats when thing go nuts. I'm thinking that you have either a battery problem, a short in your wiring, or ground issue somewhere. Take the battery out (Don't drive it until fixed or you may fry things) to your local FLAPS and they will test it for a short or dead cell for free.

I had the battery tested today and it checked out fine. What next?

voltage regulator
 

next
remove the voltage regulator. it is external on my 75 and should be on yours. it is next to the cdi box on the drivers side in the engine bay.

have it checked
if it is ok then remove the alternator and have it checked.
if all 3 are ok then you have a short.....my short was in a wire that left the alternator and should have gone to the alternator light, but was also wired to the engine fresh air blower...

hope that it is the voltage regulator or alternator.

I've had a battery with intermittent problems. That wouldn't show up in testing. The only thing that we could figure out was the panels on the inside of the battery would heat up, bend and touch one another causing the battery to short. The only way I found it was to borrow a battery, drive around and the problem went away. Check the date stamp on your battery. If you're beyond it, replace it.