Ford Fusion no start?

I have a 2008 ford fusion that I took to crappy tire because it would not start. they ran the codes, they told me a couple temperate sensors needed replaced and the car was showing like it was overheating (I said its never looked like it was overheating on the dash)according to the computer so it would automatically shut down. They cleared the codes and said the car was working and if I wanted I could take it home and see if it happens again. I couldn't even leave their parking lot without my car not being able to start again. Send it back in and this time the codes are throttle body failure and PCM needs reset. They suggest taking it to Ford because they could not do the PCM. I take it to ford, they want about 1200$ almost $800 just for the throttle body so I bought a used one, the used part has a 90 day warranty but Ford warned me that if the part was no good it would be my fault, and they would have to charge me labour twice if it didn't work. They charged me twice the labour already anyway because the put the part on, then said they had to take it off to clean the carbon out and put it on again. It worked for a week and started not starting again or starting and dying so my boyfriend suggested turning the key a couple times and stepping on the gas and then starting it saying it was sticking and Ford must of not put white grease in it. That worked great for another week and now I cant get it started at all. He thinks I should take it to ford but I'm afraid they are just going to make me replace the part again and pay them all over, but what's the chances of two bad throttle body's? Could there be another answer? it usually starts but today when I need it most it wont, I'm supposed to trade it in tomorrow at another dealership and I don't think they will want it if I cant get it started ...