dont know anyonewho had problems with the grp a mate so that should eliminate my theory

id with the alternator idea first as mentioned

also check your connectors behind the battery where the engine loom plugs in

a long shot like but on my cossie the connectors kept coming apart due to being broken clips and that gave me the same signs

crank sensor if playing up would i think play up while driving slow aswell as fast but worth a shot

oh god how i miss cossie owner ship got all this to come again soon

hope you find it mate good luck