1992 Honda Civic Smells like rotten eggs!

I have a Honda Civic 92 LX 1.5L 4door. It has 282000km. The car smells like rotten eggs here and there(not all the time)mostly when I stop(either to a light or destination). But I recently changed the catalytic converter due to rust. So I was thinking about maybe a oxygen sensor? The exhaust manifold has a really small leak caused by the gasket. The mechanic told me not to worry about the leak for now. I dont have any rpm but at idle the car seems to lope a bit. Like lets say it goes down from 1000 to 800. At intervals of 10 to 20 seconds. Im just not sure what cause the engine lope and the rotten eggs smell. Maybe the engine lope was there before and I didnt realize it since its really small.