I got my car one year ago and it's a 1981 280ZX 2 plus 2 non-turbo Edition. The car went and rode very well for the first year with only 46 original thousand miles on it. Since then I've made some modifications to it and made it a little my own and added about 10-15k MI to it. The car has worked really well over the last year however in recent times there have been some issues where it isn't so reliable all of a sudden. I worked on some dash upgrades a few days ago and got new speakers. It sounded very well and was very pleasant but 5 days later problems started to rise like no other. I drove to a place about 20 minutes away from my house where I volunteer and I noticed that my car wouldn't turn on / engage like it usually would. So I turned the key a few times and was lucky enough to get it working and went to the next location which was 5 minutes away. At this point in time, my car once again wouldn't turn on after it was shut off and my battery didn't have a good level maybe 9 to 10 volts when I tried to start it up again (multimeter). For the next 20-30 mins I tried again and again, out of nowhere got it to start up, and went home.
I gave the car a two-hour or so rest and then went to AutoZone, it started up perfectly fine and didn't really have any issues which was interesting. I got my battery tested they said it was essentially shot and I needed a new one in which case I got a new one. I bought the best one, a duralast Gold, and I went home and everything seemed fine. However, I was curious to try it out (starting my car again) and see if my car would start up again after I turned it off it didn't. At this point, I knew there was a large or underlying problem that I couldn't pinpoint so I started to chip at it.
Parasitic draw/Battery theory
My initial thought was the initial battery was drained due to parasitic draw. I tested it by taking out all of the fuses of the connections wires (chrisfix's video), yet I didn't see any change/ Outrageous DC amperage (stayed at 0). Then throughout the entire night, I tested my battery level and notice it was at a steady 12.8 V at 1-hour intervals for 5-6 hours. At which point I realized it's probably not a battery issue.
Now you might ask well maybe it's an issue with an alternator however I did get the car to start every once in a while out of pure luck at which point I would test out the and it would read anywhere from 13.6V to 14.2V constantly never going below that. And I could drive around as soon as I could get the car and it would not turn off no matter what (as far as I suspect I've driven anywhere between 15 to 20 minutes every time)
This leaves us with the starter being the possible culprit for this entire issue however I doubt that's the issue. Every time I turn the key the starter would crank and you would hear the noise of a car starting up or trying to. Now one important/interesting thing to point out was when I did get my battery checked the following day after all of these issues the auto port did say that there was not enough cranking RPM. However, I've gotten my car to start up many times since then.
Other theory (distributor)
At this point, I was really confused and I had no idea what could be going wrong as the problem persisted 50%-75% of the time. I then contacted my mechanic and he said it could be a possible issue with the distributor where things could have weird issues but I'm not too sure. When I do have the car running, I drive it like normal and I drive fast and rev high at some points of my drive. This caused no issues so I'm doubting that this is a spark plug issue. I'm not too sure if it's a distributor issue however I'm doubting spark plugs to some extent and thus I'm also the distributor.
Other interesting facts/things that I noticed
Something that I thought about when diagnosing this problem was an engine needs fuel, air, and ignition to start up. I was able to hear my fuel pump so I'm pretty sure my fuel was there however not too sure about ignition as I'm going to take a closer look at that tomorrow (sparkplugs). The only thing is I haven't gotten a new air filter and my car needs a new one because it's a little dirty, I wouldn't say horrid but dirty.
It seems like my car goes back to regular/driveable mode after I give it some time to cool down and essentially relax, but that has to be after every time I drive, even if I drive for 5 minutes around the block (no speeding, etc).
Also at one point the car would crank and engage and then die soon after (like 1 sec after) I did have my multimeter hooked on the terminals but I don't know if that would cause that issue. I have a video that I can post of that happening and the car not starting.
Video of the car starting up with a little issue
Video of the car staring and dying soon after (with multimeters touching terminals)
Any other ideas on what could be causing this issue? What do you suggest I try next?