2008 Ford TaurusX SEL (Freestyle) - a diamond in the rough or a lemon to be given away

My husband and I bought a ‘new’ used 1998 Ford TaurusX in Nov. of this past year from a guy on Craigslist (we met his wife and kids, seemed like a good guy). We loved the car but a couple of months later the engine would jump some. In Feb., I started to accelerate and it simply wouldn’t. The speedometer read ‘0’ and the check engine light went on. I coasted to a nearby store, let it rest, checked the oil – fine, transmission fluid – fine then started it up again - it made a ‘ka-chunk’ sound as I put it into gear but then ran fine. Several weeks later I noticed it sometimes lunging again. I was a bit nervous but drove it anyway. I was driving back on the highway and this time the same thing happened while I was driving 65 mph. I checked the brakes (working fine) and cruised off the next exit and parked. The car would snail forward making a whirring sound but would go in reverse with no problem. I called a tow-truck this time. Our mechanic thought it was the transmission but then he started it the next morning and said it was fine. We threw it up as another red-herring and drove it off (-$300 from our wallet). We then took it to my Dad’s mechanic, he checked it over, drove it around and could find nothing – so gave it back to us for free. A few weeks later, the same thing happened while I was driving around 30 mph. We were close to home, so I parked it in our driveway and the next morning it made the ka-chunk sound and ran fine, so I drove it to the Ford dealership. They ran a diagnostic - everything came back as great except for the Throttle Body. They said it would be $600 to fix it and should run great after that. So, we paid to have it fixed and then it did the same thing a week later. This time we had to have it towed to Ford and this time they said it was the Output Shaft Speed Sensor, which will cost $900 (and $2k if they have to go through the transmission). If that doesn’t work they will rebuild the transmission for $5k. We are now looking for a new-used car and have been told we will be lucky to get half of what we initially paid for the TaurusX. I went by the dealership to start our Ford and now it won’t even turn over the the electronic key won’t work. What gives?

@marbet if I was you, I’d have a “friendly” talk with the service manager at that Ford dealer. You paid $600 because their diagnosis said the throttle body would fix your problem. And it didn’t.
It sounds like they should be on the hook here, NOT you.