I also had a catalytic converter fail with similar symptoms... basically
the innards collapsed causing a great deal of back pressure.  May be
diagnosed by the change in the exhaust note but you haven't had yours
long enough to know what the exhaust note should be.

A reputable muffler shop can easily check for this condition -- they'll
probably drill a hole upstream from the cat and use an instrument to
measure back pressure.  Midas took care of mine though many sources are
available and (I think) a 5-year warranty on a replacement cat is
standard (maybe even the law).

Good luck.

Geo Hahn
84 Westy
Mt Lemmon, AZ