My 94 toyota corolla 214,000 miles. Recently it started skipping badly. It speeds up and slows down whenever I go above 20mph. Also the car doesnt speed up even though I press gas hard and then all of a sudden it speeds. The skipping is also making the car vibrate. I think it does or maybe it is improper balance of the engine. Could the skipping be caused by dirty spark plugs refusing to fire? Or is it because it needs an oil change? PLEASE HELP!