[Supras] Anyone have any issues with the Exide Sub

[Joel Watkinson (Cars)]

I concur. It's got to be the alternator. Or, rather, the voltage
regulator on it. IIRC, if it makes good voltage up high, but not low,
it's usually a bad regulator. In any case, I'd get another alternator,
and maybe from a different shop just so that, if you continue to have
problems, you can go to them and have it genuinely tested and not have
them just tell you "we told you the last one was fine".