My wife has a 1999 New Beetle 5-speed manual 2.0 with 110K miles.
A few times ‘only when turning right’ the battery light has come one.
It loses power (bogs) but then when straightening out after the curve all is well.
VW Dealer said battery and alternator are fine, ground is good and secure.
Firestone said the same thing.
The battery is from Sears, they said the same thing.
The engine is momentarily stalling. Usually, when this is associated with turning, it’s a fuel system problem. Occasionally, it’ll be a problem with a car’s inertial sensor. Another rare cause is a broken motor mount that allows the engine to rock in such a manner that it interferes with the engine operation.
Dealerships are not the proper place for a 1999 car. If they fixed peoples’ 1999 cars, they wouldn’t sell as many new ones. Find a reputable independently owned and operated car and give him a printout of this post, not because I’ve said anything brilliant (I haven’t), but because your description in this thread is so clear.
Open the hood and find the battery, driver’s side front of the engine compartment. It’s under a plastic cover. First make sure that it’s securely held down and that the battery cables are clean and tight. Then look at the plastic fuse holder that sits on top of the battery. They are notorious for partially melting and corroding, and may cause an intermittent momentary loss of electrical power if jostled just right.
Or it could be the engine stalling as suggested above.
Find a large open area where you can drive in circles safely. Try to duplicate the problem, if you can make it do it, note down as much as possible. Things like steering angle, speed and gear selection. If it happens again, tell her to push in the clutch to ensure it is not any type of bind in the driveline. Has the car ever been involved in an accident? I would also wiggle test the harness.
The alternator is not charging the battery properly. Check all connections and fuse. Fuse holder is known to melt. Definitely could be alternator going bad.