The car should be fixed, for reals, by the end of today. The problem was the transmission, which I was afraid of. The transmission would slip out when it idled and at times wouldn't quite catch when accelerating fast (something they caught). The total's going to be around $800, so not a cheap repair. Luckily I was able to work out a payment plan with them, so we'll be able to pay them for the repair.
I'm fed up with this car and wondering if it might be time to get a new one, but I also feel like with all the repairs and replacement parts we've added to it, maybe it'll run fine for a good long while now. It's hard to tell. Maybe, if we do get a new car, we'll keep this one as a back-up of sorts.