ther is no mention of how many miles are on this vehicle. could be worn brushes. might be poor cable contacts, im not a big believer of trying to jump power to directly to the starter, not while its installed anyway.dont know how much room you have to play with you might try tapping on the starter motor end cap, donr bang it on the sides as i dont remember when bosch introduced ceramic magnet fields that are glued to the casing. if you tap the end cap and it cranks over you may have short brushes, and would need to have the starter rebuilt. check under hood relays , fuses, or fusable links.
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mike, 1988 Volvo 240DL 114k , , previous 79 & 81 Volvo 244s