My '98 Camry (with the four-cylinder engine) usually starts, idles and drives like a dream. About a year ago I had the entire exhaust system replaced (including cat) due to corrosion. Since then it's had an intermittent problem.
Most of the time, it's fine - but every once in a while (sometimes a few times a day, sometimes several weeks between problems) it all of a sudden runs really rough - it's very noticeable at highway speeds. When it's doing this if I stop at a light it idles at a very low speed, and comes close to dying - in fact, sometimes it does die. There's a definite sulfurous "rotten egg" odor from the tailpipe when this happens. It sometimes just stops having this problem, but it usually continues until I turn the engine off. When I turn it back on a few hours later, it's always OK again. This only happens when the engine is warm.
This seems to be a "too much fuel" situation (the odor tells me this), but I'm not 100% on that. I've run some Techron through my car since this has been happening, but it still happens. My check engine light is not on. I'd start by cleaning the mass airflow sensor except my car doesn't have one (no, really!).