1998 Pontiac Grand Am sputter and no power problem when war

Well for a few weeks now when im driving anytime I get to a steady speed and hold the rpms at 1500 they will jump up and down sputtering making the car kinda buck. Well I started troubleshooting myself and I replaced the plugs and wires, there gapped right plugs in the right spot but I took it out, starting out and up throught the lower rpms it instantly had no power, it bucked, and it was like I had to force it to go but I had alot of power if I floored it and went fast. And all that was while it was hot outside. I drove for about 10 miles to base. And when I stopped I checked to see if a wire fell off and they didnt but when I leaned over the engine it was rly hot and smelled like something was burning. I checked with the shop on base and they said it sounded like the catalytic converter cus it wasnt throwing any codes, they told me to have it checked at a muffler shop but I wanted to ask u guys first. And then I went to work this morning and it was cold, probably 50s and it didnt do it at all. Im stumped and im trying to narrow it down before I spend alot of money.