that does not sound right to me. battery cables even brand new form subaru plus labor should not come any where close to 300 bucks. not only that I would think that if it were the battery cable then it would be more likely to cut out in a lower gear due to more movement in the car when driving and shifting. 5th gear seems like it would be less of a problem. what cel did you get? you said check engine light came on so what code did the "subaru mechanic" pull from the ecu? how were you driving the car when it cutout? were you flooring it or half throttle? maybe the vacuum line to the wastegate actuator has a crack in it and is allowing the car to overboost thus causing fuel cut. I need more info to try and help you.