...spk plgs cos it had cyl#1 misfire? P0301-Check engine light was on. I changed all 4 spark plugs and the light went off. Now I feel there is no power, it doesn't go beyond 45MPH. I don't feel the transmission switching to higer gears. It stalls after I brake at stops. This car was sitting idle for about 4 months and had starting problems for which I had to replace fuel pump. Oil change has not been done in the past 6 months. Could it be just this or something major?