I had my alternator replaced on my old 57 plate GS300 with a OEM Denso unit & the guy said that the after-market ones were a but hit & miss.
I had a issue on my 13 plate 250 GS where the battery terminals were a bit leaky & not easy to clamp on with the negative so wasn't getting the amp draw which affected starting which all got sorted with a new battery.
Although I swear the price was about £10 more on the battery once the cold weather kicked in Eurocar parts pure cheek !