Change the brushes on the alternator. 77K should see them fairly worn, if the spring pressure is to low they may lift/skip at speed Had same problem with a 1.9 VW engin Gal. They are a plug in module and easy to do. Alternator is at the front, may need to pop the belt off though. I found ford brushed hard to get but VW or seat will supply compatable sets. As long as the battery voltage rises a volt or so above static with the engine running thats OK. Load the electrics up and watch the lights in the dark, do they pulse on tick over? if so diode pack/regulator is suspect. Happy hunting!