1995 Civic DX acceleration problem

I've got a 5-speed Civic (135,000) that is having problems accerlating. I think it is similar to one of the accerlation questions already asked, but I thought I'd clarify for my particular situation. Ok here's the deal: SPORADICALLY, it will start misfiring (I prefer to call it "hiccupping") when I push the accerlater down any more than a gentle tap. It gets especially worse after having gone for long periods of time and at higher speeds. The oil is good, spark plugs are fairly recent (within last 10,000 miles), air filter is brand new, and I just recently had everything on the muffler system behind the catalytic converter replaced. What's my next step? [/I]