Failed smog, smoking, running rough

'91 Honda prelude 178,000 miles 2.0 SI engine - did not pass smog, would lose power for a moment after getting on the freeway, and the check engine light comes on regularly after about 15 min. Of driving. Last week, after 45 min. Of highway driving, it began idleling & running very rough. After a few more minues it was smoking slightly and dying at stop lights unless I put it in neutral. I did not use it for the rest of the week. This week it started out running fine, then was running rough, almost not going, dying at stop lights and finally smoking like a chimney. I pulled over and looked underneath and just behing the front tires, where the smoke was comming from, and the top of something (cat. Converter?) Was glowing orange and crackling. The mechanincs cannot find anything plugging the exaughst system and the car will not "run" over 15 mph to do a baseline smog test (keeps aborting). Any suggestions?