I had pretty much the same problem show up on my 86 Trooper. My tired old motor had started to run bad about a year ago, and when I checked the plugs I found it had burnt the ground electrode on #1, and since the motor burned way too much oil anyway I swapped the tired old 2.3L out for a nice 2.3L head/block I got in an Isuzu Impulse. It ran great after that other than some alternator belt squeal and the cheap old radio I had was behaving a little strange every once in a while.

Then I decided to upgrade the headlights. Those Sylvania Silverstar lights are apparently more sensitive to high voltage. They worked fine for a couple weeks, then the trouble began to show. First I blew out the highbeams taking off from a country road stop sign (hit about 6K RPMs in 2nd and poof), then the low beams died a few days later when I downshifted to pull a hill. I was thinking I got a couple bad bulbs and was pretty mad, until I got out the voltmeter and found I was seeing a bit over 15 volts at idle. When I bumped the throttle up to about 3k, it was at 19 volts and still climbing. Replaced the alt, and all was well. I'm probably lucky that I have an old 86 carb'd computerless Troop or it would have cost me more than the $50 to replace the Silverstar bulbs. I can only imagine what kind of voltage I was getting at 6k RPMS and what that would have done to the computer.

The lesson I learned? Check the voltage your alt is putting out before putting on expensive electrical parts. From $50 bulbs to expensive stereo equipment, a bad alt could cost you more than the just alternator itself.