burn 1

dont hit your starter too hard you may crack the permanent magnets inside

become friends with a subaru mechanic and i would be talking to the people who installed the battery

sounds like a strange sort of problem I would start by checking pwr and grounds and ignition switchs are weak spots in the old subs to check your alt while running use a multi meter and see what sort of voltage it produces 13.5 to 15 is good depending on the bats stat of charge just throwing parts in your car on a hunch can get costly i have been there its better if you can prove that they are defective