[papajam]

Welcome to the Board, alfamarty!
Sounds like you have two things going on. The charging system needs to put out 13.5v minimum all the time the engines running; not 'over 12'. And there's excessive voltage drop to the fuel pump. Perhaps a corroded connection somewhere? I'll have a look at the schematic. In the meantime, someone with more GTV6 experience than I will probably chime in.