2006 Pontiac Grand Prix Shifting

My car has always shifted back and forth out on the highway. This problem has always come and gone, but steadily gotten worse. Lately, my engine light came on and I read in the manual to reinstall the gas cap and the light would go off, so I did that and it worked. However, 2 days later the engine light came back on on the same day that the shifting issue was much much worse all of a sudden and was even shifting hard off of the interstate. I'm going to replace the gas cap altogether hoping this will take care of the engine light issue, but the shifting problem is worrying me and I'm really needing to know what could be causing this.