Misfires and backfires

Disconnect the small plug on the side of the generator, then run the engine to see if the symptoms clear up. If they do suspect the generator has a failed diode and is developing harmful voltage spikes. To reduce the high number of repeat failures on 1987 and newer GM vehicles, always replace the battery at the same time the generator is replaced, unless it is less than about two years old. The battery is the key component in damping and absorbing those spikes, but as they age and the lead flakes off the plates, they lose their ability to do that. Those spikes will damage the internal diodes and voltage regulator, and interfere with computer sensor signals. They are a common cause of elusive engine running problems.

Rats. The next step is to connect a scanner to view live data on a test-drive. Most have a "record" capability that allows a few seconds of data to be recorded when the symptoms occur. Because that data passes through the scanner's memory, the recording actually starts a couple of seconds before the button was pressed. The recording can be played back slowly, later, to see what changed during the event.

Wonderful news. On a lot of older cars, the engine will not run with a missing signal from either the crank or cam sensor. Some GM V-6 engines are known to not start with either signal missing, but will continue running when a sensor fails while it is running. The crank sensor tells the Engine Computer when a piston is coming to top dead center. It's the cam sensor that tells the computer which piston is coming to TDC. On some GM engines, with a missing cam signal, the computer will guess which pair of cylinders to fire first during cranking. It has a 33 percent chance of guessing correctly, and if it does, the engine will start and run. If it guesses wrong, you have to turn the ignition switch all the way off, then try again with another 33 percent chance of being right.

I don't know how you figured this out, but I'm happy you did.