Sounds to me like an alternator and/or regulator is in the process of failing based on previous experience w/ other cars.
Don't know if the two are separate on these cars, so it could be one or the other.
A multimeter check at the battery terminals at various rpms/loads should confirm failure to charge & conditions of failure.
Unfortunately, the charging system is a "closed loop" type of setup where the operation of the regulator is dependent on battery-state of charge; dependent on the ability of the alternator to charge the battery based on the condition of the battery, etc.
Various shop manuals illustrate mulimeter procedures to use so that you can narrow down the problem area....
Example: at idle, no-load voltage @ battery terminals should be about 13.5 volts; with headlights on, should be around 12.5 volts.
Use a good multimeter & not your car gauge to measure!
Good luck!
