So while coasting in gear, my car, a 99 sl1, starts to kind of feel
like it's misfiring.  if I tap the gas, it will stop, and I can
conitinue to coast, until it does it again in a few more moments.
Someone said that this could be from a faulty Catalytic Converter - is
this true?  just gave the thing a tune up hoping that was the
problem.

I haven't replaced the fuel filter; could this be it?

What else?  Any help is appreciated!