Wrong new alternator or bad voltage regulator for 4.8is

[ImolaRed48]

Hi, first post so apologise if doing anything wrong..

I bought an Imola red 4.8is recently whoch I love and will post pics.

Recently had to dive in to the nearest garage for a new alternator.

They installed a Delco Remy dra0488 Remanufactured - after being told they would source ans original Bosch - and charged me £600 for this including £55 for 1 hours labour)

Recently won't start always as it drops below 10v and sometimes below 12 whilst driving, other times it hits 18v after flooring it a few seconds. Then I'll stop at traffic lights and it will show 11v.

Luckily I have a Seuoki 2000amp jump starter of Amazon - recommended for these big engines!

Had to put in a new battery a week after this new alternator, however it was on its way out anyway.

Thoughts please on this exact model of alternator etc?

Many thanks..