1990 Porsche Carrera Battery Voltage Spikes and Secondary Ignition Questions

Hi everyone, I'm new here (this is actually my first post) and just getting started on scoping. I have a customer's 911 (964) which has drivability issues. Bad idle and sporadic stalling. Bucks intermittently while driving, intermittent split-second flashes of all the dash warning lights (like a Christmas tree), intermittent stalling. It does not feel like a one-cylinder miss; the car completely falls on its face when it happens while driving. 

After doing some basic visual checks and ruling things out in the vacuum system, I came to the battery to test it. It's weak but charging while running. No starting problems. I connected my voltage probe to the terminals, and I'm picking up some huge spikes. Down to 7V and up to 20V. 13.3-13.5V at idle. At this time, I'm thinking maybe a faulty alternator. Investigating these spikes further, I connected a secondary ignition pickup to one of the coil wires and noticed the spikes are timed with the ignition.

So, my main question is what I am seeing with these spikes. Is it noise or indicative of a problem? The battery is in the front trunk, so leads are nowhere near the engine or any ignition wires. 

Attached some screenshots of Pico scope. You can see in the first picture the fairly steady idle, then ignition cuts out. The engine starts to stumble, and you see it trying to stabilize as ignition comes back in it flares up a few times before finally stalling. 

Got the battery on charge, going to dig deeper tomorrow. Thanks for any input, I hope to get some answers on those spikes and maybe some advice moving forward with diagnostics. 

-Alex

 
 

Maybe A/C voltage leak form a bad diode in the alternator?
 A/C voltage can cause a lot of voltage issues.

Appreciate your reply. I'd still like to look into this as it's possible that's the issue. I haven't seen spikes like this while reading battery voltage both DC and also AC coupled. Alternator is buried and impossible for me to get on to the positive there to do a proper AC ripple test. I have the car benched for now while the battery charges before doing further testing. Will update later with my findings.

Continued looking into the secondary ignition system yesterday. Sprayed the wires with water and heard arcing. Found the 2 coil wires to distributor were shorting. Still hearing some that I can't see. Flat engine, 6 plugs on top, 6 on bottom, and no clearance to work with of course. Waiting on parts still. I believe the high voltage was making its way through some harnesses or ground messing with the voltage of the system overall.