1997 Hyundai Accent loss of power, clogg in exhaust system?

I put a 97 hyundai accent on the road about 7 months ago, replaced the altenator, battery, tire rods, ball joints put about a thousand into the car (but my friend only sold it for 300$) the engine itself seems fine but a few months ago a mechanic told me the catalitic converter was clogged (loss of power, couldnt go up hills. Etc) but I added this lucas oil tuneup in a bottle and it seemed to work itself out and after a week or 2 it drove back to normal.

Within the last week its starting again. When it happend the first time, I just needed to warm the car up drive it around a parking lot a bit avoid hills and it drove fine, this time, it started the same but now I smell fresh gas and the loss of power is more severe and I feel heat under my emergency break!

There are 2 differences this time, I charged the a/c and use it which effects the gas and I bought 4 spark plugs but only had limited time to use the tools and only replaced one spark plug and not the wire so here is my question.

Is it just the clogg comming back with vengence? It didnt get real bad until I replaced the one spark plug and when I feel the wire over it It vibrates or clicks and the others dont. Could it be the spark plug causing low performance and fresh fuel is getting into the exhaust and causing the heat? I dont have much money or options and it didnt pass emissions (o2 codes). But I need the car I cant give up on it right now. Is there anything I can do to make it easier on the engine? I was told by an auto parts counter person not to change the cat. Converter if I can avoid it but I cant get it to go past 40mph, what would you suggest as far as money being an option.