That mix of “works, then doesn’t” is usually a clue, not bad luck. It can be low refrigerant from a slow leak, a clogged coil, or airflow issues that cause the system to freeze and then thaw. The hiss is especially suspicious. If you want a clean diagnosis and options, check Refrigerant refill https://www.vip-servicecenter.com/services/heating-and-air-conditioner-repair – they explain repairs in plain language and can tell you whether it’s a refill, leak repair, or something else.