My car won't go over 5k in 1st and 2nd it won't go over 3k, and it has trouble going up hills?

my car was running fine, but now all of a sudden it doesn't want to go over 5k in 1st and in 2nd gear it has trouble going to 3k ect. It'll start to sputter like something is holding it back. I might of possibly put gas that was mixed with oil in it (someone gave me the gas and I thought it was 91). my car runs on premium it's a 96 eagle talon tsi awd. Also, it revs over to redline fine without any sputtering in neutral. Any ideas?