Car cant handle going faster than 40mph uphill!

I have a 1999 Honda Civic ex and I just got the catalytic converter replaced. The check engine light came on after the fix but after driving it around for about 20 miles it went off. 3 days later (today) it came back on and I had trouble going uphill on the freeway. While on the freeway it was revving up and down at the 3000 rpm mark all while only going 40mph max. Once it was on even land it went regular speed, but when I came to a complete stop the car felt like it was jerking without moving. I know it is a long shot but could you possibly know whats wrong with it?