I have a 91 turbo and at startup runs great per manual specs and alternator shows charging 14.4-14.5 volts. After about 2 minutes engine starts a surge from 1000-1500 rpm and alternator drops to 12.4 volts and stops charging. Battery is new and shows good on bench test. Cables to starter are clean and tight. Alternator voltage regulator and brushes replaced recently and tests out ok as well. No vacuum leaks observed. Problem started out of nowhere and I'm scratching my head trying to figure this one out.