Yep, definitely alternator. Spent a good week troubleshooting this issue on my rev D B4 checking sensors, timing belt, alternator voltages etc. Ty and me traced it down to the alternator after noticing it happening under load eg HID's on, running air con etc.
Just be careful with what alternator you install. I used an alternator from JDM rev a-c and the problems persisted. Once a direct repelcament was sourced all was good.