So, if I drive my car for more than an hour, then park it, it refuses to start. Starter turns, but I don't think it's getting fuel or something. If you let the car cool down though, sit for half an hour or so, it will start right back up as if nothing bad has ever happened to it. This problem started back in 09 I believe and had not been resolved because I rarely drive it far or long enough for it to be an issue. One time though I had been driving it for much longer than an hour, like possibly two, and while in the fast lane, during rush hour, on a major highway, the car starts to slow down on its own. I try to accelerate, but the RPMs don't increase, and it continues to drive slower, eventually misfires, and then stalls. Thankfully there was major construction going on, so I was able to merge onto the left shoulder, with an entire closed HOV lane and cones as a buffer. The other time it died on me was during a very stressful move. I had been driving it a few times that day, and eventually left it running as my daughter fell asleep, needed the AC, and it was just safer for her to stay there. I don't know how long I left it running. I wouldn't say more than an hour, but possibly. As I turned into our new neighborhood it started to misfire, and stalled. I coasted to the new house. Anyway, I have no idea what's wrong with this car. It originally was a v6, but had a v8 swap done in 08. Since it doesn't really do long distance travel since then, it's a rarity. I do know that it successfully drove two hours and back (after parked) in 2009. The only thing original I think is the fuel filter and fuel pump, which have 186k miles on it. Could that be it, or do you think it might be electrical? I'm currently getting 12mpg also. Not sure if that's related. For comparison, I used to get 15 city even after the swap. I have brand new spark plugs, wires, air filter, and MAF sensor.