Changed Intake Manifold Gaskets, after warm will not go past 20 MPH?

Hello. The intake manifold gaskets needed replacing because there was a coolant leak on the driver's side. After putting it back together, it runs choppy, while it's cold, but will get up to speed. After it's warm it falls flat on its keester. Won't go past 20 MPH.

After we first started it up, there was a crackling noise coming from under the car. The noise was traced to the catalytic converter. After running it for a while the noise stopped.

Yesterday, we took it to Walmart. That's about 4 miles round trip. On the way to Walmart, it got us there with no problem, but it was not up to operating temperature. However, on the way back, it just didn't have any power. We had to drive on the side streets because it wouldn't go past 20 MPH, with the pedal to the floor. Also, the catalytic converter smelled like fireworks.

I sure hope y'all can help us figure this out and quickly, or we'll be without transportation. There is no public transportation in our little rural town of Texas.