The car won't accelerate past 30 MPH, gets HORRIBLE fuel economy, and lurches and shakes when stepping too hard on the accelerator. Letting off a little seems to make the car more able to accelerate. Also, when shifting gears, the RPMs run WAY too high, and the car lunges forward when it shifts. From my understanding, this could be either the O2 sensor, or the timing chain, or both. The car has roughly 115,000, so I'm not sure. The difference in the cost of repairs is huge, so, what do you think?