I am in the process of buying a 2003 Mitsubishi Eclipse GTS 3.0 V6 convertible 5 spd from my father, who owns a used car dealership. The first time I drove it, It drove great, and there were no probelms that I could tell. I drove it for around 45 minutes no problem. On the way home, I put about 3 gallons of 87 gas in the car, because I didn't know it required premium gas. On the way home, the car died, we got it towed to a mechanic, and it needed its fuel pump replaced. Before it died, whenever I drove the car, most of the time the car SHOOK whenever I was giving it gas. Like violent shaking. After I got the fuel pump replaced, the mechanic put in about 8 gallons of 87 gas. This point I realized the car needs premium gas, so I put in about 5 gallons of 93. After more research some recommended I put in octane boost to bring up the gas to more premium standards. So I put in a whole bottle of Lucas Octane Boost. After that, it was driving alot better for about 15 minutes, no more shaking, but then it started again. A little better, but still very bad. Is the shake caused by using the 87 gas in the premium tuned engine? Or is there another underlying problem the mechanic didn't catch?