Approx. two weeks ago my alt went out. I took it back to have it repaired. It was a 65amp that was rebuilt into a 90amp, Ext regulator. It worked well for approx. two yrs before failing. I reinstalled the alt and cranked the Chevelle. It worked ok for approx. 10 minutes before becoming eratic. The alt gauge was jumping around and then it began to discharge. I made it home and plugged up my battery tender, and went to bed.The next day I went down stairs and noticed that the tender was still charging and thought that was strange. When I opened the hood and checked the alt it was hot as a firecracker. I immediately unplugged the charger and also the battery cable. I took the alt back off and carried it back to the shop and advised the tech what had occurred. I then find out the tech had put the wrong diodes in the alt which caused it to short out. They repaired the alt and I reinstalled it that evening. This time it worked fine. I also checked the wiring to make sure it had not burned or melted.
Now to the weird part. I drove approx. 40 miles this weekend to a car show. On the way the car just shut off while traveling about 50mph. After approx. 5 mins it started back up and I continued on. After about another 10 miles it again shut off. I waited another 5 mins and it cranked up and I made it to the show. Upon returning home that evening it did this three more times. I have a MSD 6AL and the internal coil in the HEI distributor. Do you think the coil is going bad or the MSD box. Could this be a direct result of the electrical issue with the alt, I have not had any issues until this alt issue. Any help concerning this matter would be appreciated.

HEI modules are pretty sensitive to "voltage" situations (low voltage is bad). I would think that the MSD box has protection on its supply circuits to prevent spikes, etc. from damaging their stuff. I'd start with testing the distributor module.

John,
What external voltage regulator are you using?
If it's the old points style, do yourself a favor and go buy a Wells VR715 electronic regulator.
Looks & hooks up the same on the outside but much more stable charging! After boiling up a battery a few years ago on one of the Chevelles, the VR715 is all I use on an external system.
Cost about $15 and available at most any parts store.

John,
What external voltage regulator are you using?
If it's the old points style, do yourself a favor and go buy a Wells VR715 electronic regulator.
Looks & hooks up the same on the outside but much more stable charging! After boiling up a battery a few years ago on one of the Chevelles, the VR715 is all I use on an external system.
Cost about $15 and available at most any parts store.

Thanks Bill, I switched over to one after I had the alt rebuilt two years ago. I always thought the MSD box would go out at once.I am probably going to move it under my dash. I will ck the coil and change the volt reg, I have a extra.