Hi, my 1991 Firebird v6 has recently developed an odd intermittent fault in the last couple of weeks.

From cold, if I turn the ignition key, the onboard voltmeter normally shows about 12.5v - it's a 2.5 year old Bosch battery

Starting the car, at least 8 out of 10 times, everything is normal.

Sometimes, when I start the car, the voltage goes to only about 10.5 and the lights will dim a little. A couple of times the car has even cut out, but will re-start, and the car will drive like this for a fair way, not seeming to discharge too badly. Eventually, and it seems especially if I drive it at a decent speed, it will suddenly jump back to 13.5volts.
Now this car has had faults with its pop-up lights recently, and the wiring condition is terrible there because the lights were badly converted to UK spec, I have found and fixed most of the bad wiring but there could still be some. Also this might sound weird but I have an idea turning off the (new Kenwood) stereo may have kicked it back to life, but it could be coincidence.
As it's an intermittent problem, I want to get an idea of the MOST LIKELY causes, and any tests I can carry out, before I spend money on a new alternator - I know they are not expensive but the postage is a killer for me - but if that's a likely cause I would rather buy one and be safe...
Anyone had similar symptoms and found the cause? Any good tips for structured troubleshooting?
Thanks
Alex

 

Not the same year but will give you an idea of troubleahooting guidelines.

https://www.thirdgen.org/forums/tech-general-engine/613497-1988-firebird-service-manual.html

I would check that the exciter wire is in good condition and voltage is present at alternator.

 

There is a little wireing block just to the side of your battery, it will have a lead going from the battery positive (or it may run to starter) to this and a bunch of other red wires with fuse able links going to it. It's not too uncommon for this to get corroded. Mine did and caused all sorts of issues. Also make sure the ground strap between engine and frame is good.

The next thing I would check is all the wires at the starter, make sure clean and not on exhaust.

In the states you can take an alternator to almost any parts store and they will test for free, is that an option? Also several places can rebuild them in house, may be cheaper than shipping one.