In 1981, I bought my very first, brand new, new car smell, Ford F-150.
The 300 CID engine is always, always bullet proof. Well, I just got a bad one. It had a knock from day one and I wasn't going to be satisfied making payments on a knocking truck, supporting a wife and two kids (at the time).
Looooong story short. After way too many trips, leaving the truck, "truck's fixed", (truck wasn't fixed). In 4 months, I only put 600 miles on it. I finally did a slow burn. I was making payments on a truck I didn't have.
Met my salesman, looked him in the eye and said, "I'm not leaving without a new truck". He could see I meant every word. He went off to meet with the big boss. A few minutes later, the deal was done. I could go out to the lot and pick out a new, same color truck. The new one had a cheaper seat. They swapped it for my old, better one. The new one had an automatic. OK. I'd have to pay $550 up or whatever it was at the time. Had to pay the 4% sales tax, because it is considered a new sale. OK by me. The whole deal ended up changing my payment something like 19 bucks a month. I left in the new one, and the old one was still in pieces in his back room.
BTW, that new F-150 never darkened the door of any shop, ever, and was one of the best vehicles I ever owned, flat out. Memory of the first one faded away.
At some point, and only you can decide when that is, if they cannot fix the old one, it'll be time to cut your losses and move on.