How does it idle? Run normal otherwise (no noticeable lunging/bucking)? If it's operation is relatively normal other than this, we can throw out the whole idea of plugs/wires (tends to be very consistant issues resulting from this, not occasional, and causes rough idle/running). Opti is likely not the cause either, as that also tends to be consistent issues that arise as symptoms, not occasional.
Does it typically happen when going over bumps/rough road patches? Sometimes a loose cable will cause those problems.
Next thing: when signals (turn signals. Oddly they seem to draw a LOT of current on T/A's) are turned on, do you notice the voltage gauge moving a lot (say, more than 2-3V)?
I had a similar problem with my T/A, turns out the battery cables were loose (this was just a couple days after buying the car) and when I went over bumps, it stalled out, then (still in gear. T56 6-speed BWA) a second or two later, came back. Could also possibly be the cables to/from the alternator not tightened down all the way, and it's losing contact there.
Next thing to check (if performing repairs yourself) is all the grounds/connections. It's easy work, just a wire brush, towel you don't mind getting greasy, a few wrenches and a little time. After 16yrs, cables rust, and you'd be surprised how much that can hurt the performance.
Other than that, I really don't know what to say. It's most likely going to be one of those things "you just have to be there" to diagnose. I've seen it all, from a rat in the exhaust of a '73 nova causing it to stall (too much backpressure) to a weird handling issue turning out to be an anti-roll bar that snapped in at least 5 places (several sections were missing!)
Tuesday, October 19th, 2010 AT 4:48 PM